Abstract: This study presents the design, key component optimization, and field validation of the 4LQ-1 side traction scallion combine harvester, developed to address critical challenges in mechanized scallion harvesting, such as soil clogging and crop damage, amid the large-scale development of the scallion industry where manual and semi-mechanized operations are no longer sufficient. The harvester integrates five core subsystems: a depth limiting device, a combined excavation device, a conveying and soil-cleaning device, a lateral laying device, and a transmission system, enabling sequential operations including ridge-side soil separation, excavation and lifting, clamping and conveying, rotary soil cleaning, and lateral laying in a single pass. Its main technical parameters include an overall dimension of 3600×2100×1400 mm (length×width×height), a matched power of no less than 88.2 kW, a working width of 0.25 m, a total weight of 436 kg, a three-point hitch connection, and a working speed range of 0.3-0.4 m/s. The combined excavation device, a key part of the harvester, consists of a soil separation mechanism and an excavation-lifting mechanism: the soil separation mechanism uses rotating spiral blades (with a maximum diameter of 250 mm and a shaft diameter of 75 mm) to cut, crush, and push soil from both sides of the scallion ridge, reducing the soil volume handled by the excavation shovel and preventing clogging; the excavation-lifting mechanism adopts a trapezoidal digging shovel (275 mm in width and 120 mm in length) to minimize excavation resistance, paired with lifting bars (250 mm in length and 75 mm in spacing) that loosen and lift the soil-scallion mixture, allowing crushed soil to fall through the bar gaps during lifting. The conveying and soil-cleaning device comprises a clamping and conveying mechanism and a rotary soil-cleaning mechanism: the clamping mechanism uses flexible foam rubber belts (100 mm in width) to gently grip the scallion at the junction of leaves and the white part (scallion bulb), with a feeding inlet width of 300 mm (matching the average leaf spread of mature scallions) and a depth of 600 mm, ensuring stable clamping without damaging the crop; the rotary soil-cleaning mechanism employs rubber rods rotating at 20 r/s to strike and remove adhering soil from scallion roots, balancing cleaning efficiency and crop protection (avoiding damage caused by excessive rotation speed). The transmission system draws power from a tractor: the tractor’s power take-off shaft connects to the harvester’s main drive shaft via a universal joint and an HD input commutator, powering the soil separation mechanism (through an HD output commutator and chain drive) and the rotary soil-cleaning mechanism (through chain drive), while the tractor’s hydraulic system drives the clamping belt’s hydraulic motor and lifting cylinders, and the lateral laying device is powered by the harvester’s own electrical system. To optimize performance, a three-factor, three-level orthogonal experiment was conducted, with forward speed, soil separation mechanism rotation speed, and clamping belt inclination angle as variables, and leakage rate (unharvested scallions), damage rate (structurally damaged scallions), and excavation rate (successfully harvested scallions) as evaluation indicators. The experiment was carried out in a sandy loam field in Jiaozhou, Shandong Province, in May 2024, where soil moisture content was 8.3%, soil compaction was 0.6 MPa, and scallions were planted with 269 mm ridge height, 801 mm ridge spacing, and 62 mm plant spacing. Using Design-Expert software, mathematical models between factors and indicators were established, and the optimal parameter combination was determined: a forward speed of 300 mm/s, a soil separation mechanism rotation speed of 200 r/min, and a clamping belt inclination angle of 30°. Field validation tests (three replicates, each covering 100 m of ridge) showed that under these parameters, the leakage rate was 0.8%, the damage rate was 2.85%, and the excavation rate was 98.7%, with the absolute error between measured values and model predictions within 5%, meeting the technical requirements for scallion harvesting equipment (leakage rate ≤5%, damage rate ≤5%, excavation rate ≥90%). This research provides theoretical basis and technical support for the structural improvement and performance enhancement of side traction scallion harvesters, effectively reducing harvesting costs and improving operation quality for ridge-planted scallions in China.