Nitrogen (N) and phosphorus (P) are important factors controlling biogeochemical cycling in terrestrial ecosystems and significantly affect the decomposition process of litter. However, N and P addition effects on litter decomposition, especially biological pathways in alpine grassland in northwest Sichuan, remain unclear. Therefore, this study explored the response mechanism of Salix cupularis litter decomposition and soil enzyme activity to different exogenous nitrogen and phosphorus additions and the inner relationship, combined with field in situ experiments and laboratory analysis methods. The results showed that: (1) In general, N, P and NP treatments significantly promoted the decomposition of litter, lignin and cellulose, and the addition of NP had a stronger effect in the same concentration level. P addition could alleviate the inhibitory effect of litter decomposition by high N concentration. (2) Litter C, N and P basically showed a sustained release pattern, and NP treatment had the strongest promoting effect. (3) Each treatment significantly increased the activities of soil invertase, cellulase, polyphenol oxidase, urease and phosphatase, and the NP treatment had the best effect. The rate of litter decomposition was significantly influenced by nutrient content as well as soil enzyme activity, where cellulose content and invertase activity may be a key factor controlling the rate of litter decomposition.