The immune function of animal is an important factors to influence animal  fitness. As it is a crucial way to defend against bacterial pathogens such as viruses, the immune system play a key role in animal’s survival. Both population density and parasitization are known to deteriorate individuals’ immune function. This experiment using 2×2 factorial experiment design to measure the indexes of four treatments(maternal density stress+coccidian infection, not maternal density stress+not coccidian infection, maternal density stress+not coccidian infection, not maternal density stress+coccidian infection), including the fecal corticosterone metabolite(FCM), coccidian infection rate,blood parameters, lymphocyte subsets counts(CD4, CD8 counts). The results show that the density-induced maternal stress can significantly reduce the number of CD4, rate of CD4/CD8, Lymphocytes, Eosinophils, white cell. The effects were further deteriorated when maternally stressed offspring infected with coccidian. Our results showed that the interaction of density-induced maternal stress and coccidian infection has negatively synergistic effect on the immune of root vole．