Eucalypt (Eucalyptus grandis) is widely used to develop the short-term rotation industrial plantation in south China due to the characteristics of fast growth, higher yield, higher cellulose content, and straight trunk. However, extensive eucalypt pure plantation gives rise to a series of ecological problems such as native biodiversity loss, soil fertility decline, "green desert", and plant disease outbreak. In order to avoid or lower the ecological and economic risks of pure plantation, therefore, forest managers and researchers begin to plant the mixture of eucalypt tree with native trees. Alder (Alnus formosana) is a nitrogen-fixing tree species, which can increase soil fertility and accelerate the decay of lower-quality litter and nutrient cycling, implying that the alder may become the accompanying tree species in the eucalypt plantation. Meanwhile, soil faunal community plays important roles in maintaining soil fertility, decomposing plant residues and other soil processes. In order to evaluate the effect of mixed eucalypt and alder plantation on soil ecological process in the hilly region of Southwestern Sichuan, therefore, 10-g litters with the ratios of eucalypt to alder 10:0 (TI), 7:3 (TII), 5:5 (TIII), 3:7 (TIV) and 0:10 (TV) were kept in nylon bags with the sizes of 260-mesh, 30-mesh and 6-mesh, and were placed on the forest floor in the plantations with different ratios of eucalypt and alder trees, respectively. Consequently, the structure and composition of soil faunal community during the decomposition of mixed eucalypt and alder litters were investigated from May 2009 to April 2010. Soil macrofauna in litters was picked up by hand in the fields. Mesofauna and microfauna were collected and separated from the soil samples by Tullgren methods. 75651 specimens, which belong to 2 phyla, 10 classes and 20 orders, were found in five mixed leaf litters and three sizes of litterbags. Acarina and Collembola were the dominant groups, accounting for 97.24% of the total. The litters had the maximum peak of soil faunal quantity from July to August. Similarly, the maximum peak of soil macrofaunal quantity was observed in July, and that of meso- and micro-fauna in July and August, respectively. No obvious difference on the faunal groups was found among months. More meso- and microfaunal individuals existed in 260-mesh litterbags than in 30-mesh and 6-mesh litterbags. Eucalypt litter had higher amount of Collembola in comparison with the alder litter. TV litter had more Psocoptera compared with the others. Mixed litter had more Opisthopora. The structure of soil faunal community in litters varied with the mixed ratios of eucalypt to alder litters. These results implied that the soil macrofauna prefers to live in the alder litter and mixed litters rather than the eucalypt litter, and accelerates the rate of leaf litter breakdown. Therefore, alder litter and mixed litters had higher decomposition rates compared with the eucalypt litter. These results demonstrated that the alder was an accompanying tree species in the eucalypt plantation, and alder trees planted in the pure eucalypt plantation will be of benefit to accelerating the decay of eucalypt leaf litters and to maintaining the soil fertility. The results provide with scientific basis for reasonable mixed eucalypt plantation.