Litter decomposition is an essential process of material cycling in the terrestrial ecosystem that can be the primary source of nutrients for plant growth, and of both nutrients and energy for soil biota. The present consensus is that climate-related precipitation and temperature patterns determine the rate of litter decomposition at the regional scale, while the substrate quality related to plant species manipulates the process of litter decomposition at the ecosystem level. Consequently, much more attention has been focused on litter decomposition as affected by climate, substrate quality, and soil biota in past decades. Theoretically, precipitation, temperature, biological activity, and their combined effects control the process of litter decomposition at different critical periods. As yet, little information has been available on the process of litter decomposition in different rainy and dry periods. Therefore, in order to understand the process of litter decomposition in seasonal rainy and dry periods, a field litterbag experiment was conducted in the Chongzhou Modern Agricultural Research & Development Base of Sichuan Agricultural University, which is located in the subtropical humid climate zone of Sichuan Basin at the upper reaches of the Yangtze River. Foliar litter was selected from Pinus massoniana, Cryptomeria fortunei, Cunninghamia lanceolata, Cinnamomum camphora, Toona ciliata, and Quercus acutissima, which are widely distributed in the subtropical evergreen broadleaved forest. Litterbags were sampled in the dry season, spring minor rainy season, early rainy season, later rainy season, and winter minor rainy season as litter decomposition proceeding from January 18, 2013 to January 18, 2014. In turn, the mass loss rates of foliar litter were measured, and rainfall and temperature were investigated at the same time. The results indicated that the litter mass loss of all six tree species increased as decomposition proceeded. Over one year's decomposition, the remaining litter mass of six tree species was in the order T. ciliata (27.90%) < C. fortunei (41.39%) < C. lanceolata (48.93%) < Q. acutissima (49.62%) < P. massoniana (68.82%) < C. camphora (72.23%). Seasonal rainfall had significant (P < 0.05) effects on the litter mass loss of the six species. Compared with needle litter, broad-leaved litter had higher mass loss in the dry season (MRS, SRS, and WRS). The mass loss in the rainy season (ERS and LRS) accounted for 69.73%-89.68% of the mass loss of the entire first decomposition year, while that in the dry season accounted for only 10.32%-30.27%. Meanwhile, the rates of foliar litter mass loss in the rainy season were significantly (P < 0.05) higher than those in the dry season regardless of tree species. Pearson correlation analysis results revealed that the litter mass loss rate was significantly (P < 0.01) and positively correlated with precipitation and temperature. In addition, mass loss was significantly (P < 0.01) and negatively related to the initial concentrations of C, lignin, C/N, and lignin/N in foliar litter, but significantly (P < 0.01) and positively related to the initial N concentration in foliar litter. In conclusion, the mass loss of foliar litter occurred mainly in the rainy season in the subtropical humid region, so changes in seasonal rainfall patterns and the corresponding changes in temperature and humidity would significantly affect the process of litter decomposition.