This study focuses on the acetylation modification of polysaccharides from Rhododendron dauricum leaves (RDPs) with a high degree of substitution (DS) and then discusses their characterization and biological activity. The optimum acetylation conditions of RDPs were optimized by response surface methodology, which were reaction time 3 h, reaction temperature 50 °C, and the liquid-solid ratio 16 mL/g. Under the optima schemes, two eco-friendly acetylated polysaccharides from R. dauricum leaves (AcRDP-1 with DS of 0.439 ± 0.025 and AcRDP-2 with DS of 0.445 ± 0.022) were prepared. The results of structural characterization showed that the AcRDP-1 (9.3525 × 10³ kDa) and AcRDP-2 (4.7016 × 10³ kDa) were composed of mannose, glucose, galactose, and arabinose with molar ratios of 1.00:5.01:1.17:0.15 and 1.00:4.47:2.39:0.88, respectively. Compared with unmodified polysaccharides, the arabinose content and molecular weight of the two acetylated polysaccharides decreased, and their triple helix conformation disappeared, and further improved their anticomplementary activity. The two acetylated polysaccharides showed stronger a complement inhibition effect than the positive drug by blocking C2, C3, C4, C5, C9, and factor B targets in the classical and alternative pathways. This research indicated that acetylation modification could effectively enhance the anticomplementary activity of RDPs, which is beneficial for the development and utilization of R. dauricum leaves.