Litter, an essential component of the forest ecosystem, plays an important role in maintaining site fertility, sequestering carbon, and nursing soil biodiversity. However, the substrate quality of decomposing litter is affected by increasing nitrogen deposition mainly because of fossil fuel combustion and chemical fertilizer production and use. Theoretically, decomposition of atmospheric nitrogen may have a strong impact on litter decomposition in three ways. The first approach is that nitrogen deposition may alter the chemical components of litter by direct nitrogen addition. The second approach is that nitrogen addition can change the growth of plants and carbon and nutrient allocations in plant tissues and indirectly lead to changes in litter substrate quality. The third approach is that nitrogen deposition may result in soil acidification, and, in turn, have strong effects on litter substrate quality indirectly. To date, there is limited information on the changes in litter substrate quality due to atmospheric nitrogen deposition in the Rainy Area of Western China. To understand the effects of increasing nitrogen deposition on the litter decomposition process in natural evergreen broadleaved forests in the Rainy Area of Western China, a field litter decomposition experiment was conducted in a natural evergreen broad-leaved forest in Ya'an City. Between November 2013 and November 2014, we conducted the field experiment by using the litterbag method. Four levels of nitrogen deposition were set:control (0 kg N hm^-2 a^-1), low nitrogen (50 kg N hm^-2 a^-1), medium nitrogen (150 kg N hm^-2 a^-1), and high nitrogen (300 kg N hm^-2 a^-1). The results indicated that nitrogen deposition significantly influenced litter substrate quality, and concentrations of carbon, nitrogen, and cellulose in the leaf litter were found to increase significantly with nitrogen addition. Nitrogen deposition also affected C/P and C/N of the leaf litter; all nitrogen deposition treatments significantly increased C/P, whereas medium nitrogen and high nitrogen treatments significantly increased C/N. Since nitrogen deposition significantly increased the nitrogen concentration of the litter, exacerbating the accumulation of litter lignin and cellulose, after decomposing for 1 a, lignin/N and cellulose/N after each nitrogen deposition treatment were significantly higher than those in the control. The correlation coefficient of mass remaining of foliar litter with C/N, lignin/N, and cellulose/N was increased by nitrogen deposition, while C/P was reduced. Thus, simulated nitrogen deposition influenced the substrate quality of foliar litter in natural evergreen broad-leaved forest in the Rainy Area of Western China, and influenced the decomposition process of the litter.