Extracellular enzyme plays a crucial role in litter decomposition for the degradation of organic matter. In the course of litter degradation, enzyme activity is not only influenced by the litter species or substrate quality, but also affected by environmental factors. Invertase catalyses the hydrolysis of sucrose to glucose and fructose; thus, invertase is more essential than enzymes related to recalcitrant material degradation in the early litter decomposition period. We investigated the invertase activity of 12 representative leaf litter in alpine timberline ecotone of western Sichuan, China. The effects of species and environmental factors on invertase activity were also studied. The results showed that invertase activities of 12 species were significantly different under the same vegetation type (P < 0.01). The effects of species, environmental factors, and their interactions on invertase activity were highly significant (P < 0.01). In addition, the relationship between the initial cellulose content and invertase activity was positively and extremely significant (P < 0.01). Moreover, there was a significant and negative correlation between invertase activity and initial lignin and total phenol content, respectively (P < 0.01). The lignin and total phenol content can explain the 50.8% variation in invertase activity. Invertase activity of graminoids was the highest among all plant life forms, which likely occurs due to the higher initial cellulose content and lower lignin and total phenol content of graminoids. Multiple linear regression analysis showed that leaf litter moisture can explain the 62.1% variation in invertase activity and was the most important environmental factor variable. From the perspective of vegetation type, the invertase activity of most species in the coniferous forest was significantly higher than that of species in the alpine meadow and shrubs (P < 0.01), which may be caused by the highest litter moisture content and snow depth in the coniferous forest. After a snow cover period decomposition, the comprehensive effect of leaf litter quality and environmental factors on invertase activity can explain the 79.1% variation in invertase activity, indicating that invertase activity was mainly controlled by lignin, total phenol content, and leaf litter moisture in the alpine timberline ecotone of western Sichuan. In the global climate change scenario, the change in moisture content of the litter will strongly influence invertase activity in the early stage of litter decomposition.