Litter decomposition is a key process to maintain nutrient circulation and energy flow in the ecosystem. However, in the loess hilly region where rain and heat happen in the same period, it remains unclear how the litter quality during different rainfall periods affects litter decomposition rate. In order to better understand the leaf-litter decomposition characteristics during different rainfall periods and the relationships between the dynamic in substrate mass and decomposition rate during decomposition, a field litter bag experiment was conducted in two types of planted forest of Robinia pseudoacacia and Pinus tabulaeformis in the loess hilly region. The results revealed that: (1) after 391 days of decomposition, the average leaf-litter mass loss rate of R. pseudoacacia was (51.0±8.44) mg/d, significantly higher than that of P. tabulaeformis (36.7±4.83) mg/d. The leaf-litter mass loss rate of both species during rainy season was significantly higher than those during dry season. (2) During decomposition, C and N content in the leaf-litter of two studied species showed a net release, mainly occurring in the summer rainy season. The dynamic of P content showed a release and enrichment alternately. The leaf-litter C/N, C/P and N/P ratios of R. pseudoacacia showed a fluctuant trend, while the C/N ratio of P. tabulaeformis increased significantly over time and peaked in summer rainy season, the C/P ratio had a trend of fluctuation but the N/P ratio did not significantly change. (3) The leaf-litter mass loss rate of R. pseudoacacia was significantly and positively correlated with leaf-litter P content dynamic, negatively correlated with C content, C/P ratio and N/P ratio. However, the leaf-litter mass loss rate of P. tabulaeformis was positively correlated with C/N ratio and negatively with C and N content and N/P ratio, and showed negative quadratic function with N/P ratio. These results indicated that the leaf-litter decomposition of R. pseudoacacia and P. tabulaeformis in loess hilly region tended to significantly differ in different rainfall periods and mainly occur in rainy season. And the leaf-litter decomposition was mainly restricted by leaf-litter N content and N/P ratio, the leaf-litter decomposition of P. tabulaeformi was more restricted by N content and N/P ratio than that of R. pseudoacacia.