<p>Mild alkaline pretreatment with different NaOH dosages (1–5%) was applied to rice straw (RS) for saccharification enhancement and lignin production. The NaOH concentration effect on the RS composition and enzymatic hydrolysis performance was first evaluated. The pretreated RS reveals a saccharification yield (glucose, xylose) of 47–50%, showing significant improvement compared to the raw RS and hydrothermal pretreated one. Increasing NaOH concentration enriches cellulose and enhances the overall sugar recovery, while 2% NaOH shows the optimized enzymatic saccharification performance, with hydrolysis efficiency for glucose and xylose reaching 64% and 41%, respectively. Acid precipitation (H₂SO₄) was used to recover alkali lignin from the pretreatment hydrolysate (black liquor). The derived crude alkali lignin showed a purity of 45.3–59.5%, further improved to 69.8–81.3% by acid hydrolysis. Alkaline solution pH shows a well-linear correlation with the composition of pretreated RS biomass and crude alkali lignin, especially for xylan, Araban, and acid soluble lignin (ASL), with linear regression coefficient (<i>R</i>²) of 0.989–0.991, 0.991–0.996, and 0.932–0.999, respectively.</p>