In order to understand the decomposition characteristics of eucalyptus (Eucalyptus grandis) -alder (Alnus formosana) mixed litters and the roles of soil fauna in the decomposition process, litter samples of the two tree species were collected from the Suji Town of Leshan, Sichuan Province, and mixed with different proportions. The mixtures were put in 6-, 30-, and 260-mesh litterbags to investigate their mass loss and the dynamics of soil faunal community during the process of decomposition. Different proportion mixtures all presented the same decomposition pattern, i. e., decomposed rapidly in early period and slower in later period. The mixtures had the highest decomposition rate in 6-mesh litterbag, followed by in 30-mesh, and in 260-mesh litterbag. In the litterbags with same meshes, the decomposition rate of different proportion mixed litters also varied. In 6-mesh litterbag, the durations for the decomposition of the litters had slight difference; while in 30- and 260-mesh litterbags, the durations for pure E. grandis and A. formosana litters getting 95% mass loss had a difference of 1175 and 908 days, respectively. During the decomposition of the litters, soil macrofauna had an obvious change in community structure. In early period, Psocoptera dominated; in mid period, Opisthopora dominated; in later period, Coleoptera dominated; and by the end, Diptera dominated. This study provided important information to understand the material cycling in eucalyptus-alder mixed plantation.