Rice straw can serve as a promising feedstock for fermentable sugar production, due to its abundant and carbohydrate-rich biomass. However, its complex lignocellulosic structure has hindered direct saccharification. Therefore, it is often required for environmentally friendly and energy-efficient pretreatments to disrupt the dense structure and selectively fractionate lignocellulosic components for the high sugar yield. In this study, a combined Na₂CO₃-freeze and hydrothermal pretreatment was applied to rice straw. Single-factor experiments were conducted to investigate the effects of freezing parameters (times and temperature), Na₂CO₃ concentration, hydrothermal treatment time, and solid-to-liquid ratio on structural deconstruction, physicochemical properties, and saccharification efficiency. Enzymatic hydrolysis conditions (solid loading, enzyme loading, and Tween 80 concentration) were optimized using a central composite design (CCD). High-solid saccharification was achieved at low enzyme loading using a fed-batch strategy. The results showed that the optimal pretreatment conditions were determined as: 0.6 mol/L Na₂CO₃, a solid-to-liquid ratio of 1:15, two freeze times at -20 ℃, hydrothermal treatment at 121 ℃ for 30 min. The rice straw shared a solid recovery of 61.1% after pretreatment, 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, while the lignin removal reached 74.8%. After enzymatic hydrolysis （5% substrate, 10 FPU/g cellulase (Cellic® CTec2), pH 4.8, 50 ℃, 160 r/min, 72 h）, glucose and xylose concentrations of 28.8 and 8.9 g/L were recorded, respectively, markedly exceeding the 2.3 and 0.3 g/L from the untreated control. The corresponding yields of 98.1% and 68.5% represented an 8.0-fold increase for glucose and a 27.5-fold increase for xylose, respectively, compared with the untreated straw. The samples were characterized via scanning electron microscopy (SEM), N₂ adsorption-desorption (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). It was found that the combined pretreatment effectively disrupted the physical barrier and lignin-carbohydrate network, while selectively removing lignin for the high specific surface area, pore volume, and pore size. Enzyme accessibility was enhanced after structural modifications. Furthermore, the parameters of enzymatic hydrolysis were further optimized using the response surface method (RSM). Analysis of variance (ANOVA) confirmed that there were strong correlations between predicted and experimental values, indicating the high reproducibility and reliability. The optimal combination of parameters was 8% substrate, 8 FPU/g cellulase, and 110 mg/g Tween 80. As such, 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 and 14.3 g/L over untreated rice straw. Finally, high-solids 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 and 33.0 g/L, with the yields of 63.3% and 42.4%, respectively. The alkaline salt-freeze combined with hydrothermal pretreatment can be expected to serve as a promising strategy for high-solids enzymatic hydrolysis of lignocellulosic biomass at low enzyme loadings. These findings can also provide a strong reference for the sustainable and energy-efficient production of fermentable sugars from agricultural residues.