Abstract: Rice straw is produced in large quantities and is rich in carbohydrates, making it a promising feedstock for fermentable sugar production. However, its complex lignocellulosic structure hinders direct saccharification. To address this, environmentally friendly and energy-efficient pretreatment methods are needed to disrupt the dense structure, selectively fractionate lignocellulosic components, and improve sugar yield. In this study, a combined Na₂CO₃-freeze and hydrothermal pretreatment was applied to rice straw. The effects of freeze parameters (times and temperature), Na₂CO₃ concentration, hydrothermal treatment time, and solid-to-liquid ratio on structural deconstruction, physicochemical properties, and saccharification efficiency were investigated through single-factor experiments. Enzymatic hydrolysis conditions (solid loading, enzyme loading, and Tween 80 concentration) were optimized using a central composite design (CCD), and high-solid saccharification at low enzyme loading was achieved through a fed-batch strategy. The optimal pretreatment conditions were determined as: 0.6 mol/L Na₂CO₃, solid-to-liquid ratio of 1:15, two freeze times at -20℃, hydrothermal treatment at 121℃ for 30 min. Under these conditions, the pretreated rice straw had a solid recovery of 61.1%, composed of 52.9% cellulose, 27.5% hemicellulose, and 12.3% lignin. The recoveries of cellulose and hemicellulose were 97.6% and 87.5%, respectively, and lignin removal reached 74.8%. After enzymatic hydrolysis 5% substrate, 10 FPU/g cellulase (Cellic® CTec2), pH 4.8, 50℃, 160 rpm, 72 h, glucose and xylose concentrations of 28.8 g/L and 8.9 g/L were recorded, markedly exceeding the 2.3 g/L and 0.3 g/L from the untreated control. These values correspond to yields of 98.1% and 68.5%, respectively, representing an 8-fold increase for glucose and a 27-fold increase for xylose compared to untreated straw. Characterization via scanning electron microscopy (SEM), N₂ adsorption-desorption (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) revealed that the combined pretreatment effectively disrupted the physical barrier and lignin-carbohydrate network, selectively removed lignin, increased specific surface area, pore volume, and pore size, and reduced thermal stability. These structural modifications enhanced enzyme accessibility and subsequent hydrolysis. Enzymatic hydrolysis conditions were further optimized using response surface methodology (RSM). Analysis of variance (ANOVA) confirmed strong correlations between predicted and experimental values, with high reproducibility and reliability. The optimal conditions were 8% substrate, 8 FPU/g cellulase, and 110 mg/g Tween 80. Under these conditions, 72-h enzymatic hydrolysis yielded glucose and xylose concentrations of 44.8 g/L (95.3% yield) and 15.3 g/L (73.6% yield), respectively, corresponding to increases of 38.6 g/L and 14.3 g/L over untreated rice straw. Finally, high-solid saccharification was achieved at 8 FPU/g enzyme loading and 30% solid loading using a fed-batch strategy (initial solid loading of 8% with 110 mg/g Tween 80, followed by additions of 8%, 8%, and 6% substrate at 4, 8, and 12 h). After 72 h, glucose and xylose concentrations reached 111.7 g/L and 33.0 g/L, with yields of 63.3% and 42.4%, respectively. These results demonstrate that alkaline salt-freeze combined with hydrothermal pretreatment is a promising strategy for achieving high-concentration enzymatic hydrolysis of lignocellulosic biomass at low enzyme loadings. This study provides a novel approach for the environmentally sustainable and energy-efficient production of fermentable sugars from agricultural residues.