Abstract: Flax is an important oil crop that is widely cultivated in the dry farming areas of Loess Plateau in Northwest and North China. The seed of flax is rich in high nutrition with many beneficial effects, such as anti-inflammatory, anti-cancer, and regulating blood lipid. A staged harvest is generally used to collect the flax seed in previous manual operations in remote mountainous areas. However, high labor intensity and low operating efficiency have seriously restricted mechanized harvesting in the sustainable development of the flax industry. Since Gansu Province is one of the main hilly planting areas of flax, large combine harvesters cannot run smoothly in the operation of transportation, field entry, and transfer, due mainly to the small blocks and narrow roads. In this study, a track combine harvester was designed for hilly mountain flax, combining with easy winding, high strength, and low mobility of flax stem. This machine included a crawler-type walking system, a low damage header to prevent winding, a transverse-flow beater with the grain rod and rod teeth with small taper, narrow-grid concave plates, and secondary cleaning. The whole feeding realized the joint harvest of flax in hilly areas, including dividing, cutting, conveying, threshing, separating, and cleaning. The crawler walking system adopted hydraulic steering and variable speed, showing the 360° in-situ steering with a turning radius of only 700 mm. In the anti-winding low damage header, an anti-winding plate was set in the feed section of the agitator wheel, while the number of telescopic teeth was reduced. As such, the rotational speeds of the threshing and agitator wheel were adjusted to realize the anti-winding of the flax stem. The threshing roller included the transverse-flow beater with the grain rod and rod teeth with small taper, thereby realizing the complete threshing and separation of the flax capsule. The secondary cleaning was composed of a large air-duct fan, a centrifugal fan, and a cyclone separator to implement two cleaning for flax threshing materials. A field experiment was carried out to evaluate the performance of the combine harvester. The results showed that when the water content of flax seeds was 5.42%, an optimal combination of parameters was achieved, where the threshing rate was 98.76%, the impurity rate was 3.61%, the damage rate was 0.18%, the header loss rate was 1.07%, the entrainment loss rate was 0.25%, the cleaning loss rate was 0.81%, the splash loss rate was 0.26%, and the total loss rate was 2.36%. The whole machine ran smoothly during the operation. The operation indexes fully reached the mechanized harvest requirements of flax to serve as a hilly flax combine harvester.