Forest gaps are an essential driver of litter decomposition in alpine forest ecosystems, but how gap size influences lignin and cellulose degradation in twig litter remains poorly understood. Here, we conducted an 8-year decomposition experiment to assess twig litter decomposition (i.e., mass loss, cellulose and lignin degradation) in four gap size classes in an alpine forest of Southwest China, including (1) large gaps (15–20 m in diameter), (2) medium gaps (10–15 m in diameter), (3) small gaps (<10 m in diameter) and (4) closed canopies. After 8 years of decomposition, the remaining mass, lignin and cellulose of twig litter was 32.71–36.85%, 39.82–49.84% and 15.77–21.34% across the gap size classes, respectively. Compared to the closed canopy, forest gaps decelerated the decomposition processes of twig litter, with significantly higher remaining of mass, lignin and cellulose of twig litter observed in the small gaps. However, changes in the gap size from small gaps to large gaps did not affect the decomposition rate or lignin and cellulose degradation of twig litter during the study. Furthermore, gap-induced changes in litter chemistry (i.e., N and P contents, C/N and lignin/N ratios) and microclimate (i.e., soil temperature and freeze–thaw events) had significant effects on the lignin and cellulose degradation of twig litter. These results indicate that the formation of gaps might slow the lignin and cellulose degradation of twig litter and that gap effects vary with changes in litter chemical properties among different litter substrates (i.e., leaves and twigs) in alpine forests.