Both visceral organ mass and body mass show phenotypic plasticity in small mammals, which in turn is related to their living environmental conditions. To further understand the seasonal adaptive strategy for wild Red-bellied Squirrel (Callosciurus erythraeus), 174 males and 136 females were captured from January to December 2015, respectively from Yingjing County, Sichuan Province. We measured body mass, wet masses of visceral organs (liver, heart, spleen, lung, kidney, testis, uterus), analyzed the data from different sexes and seasons. Besides, the impact of pregnancy in body mass and wet masses of visceral organs were also analyzed. These indices were analyzed by Two-way ANOVA or ANCOVA. If some indices showed significant sex differences, seasonal variations of those indices for any gender were further analyzed by one-way ANOVA or ANCOVA. The results were as follows: 1) No sex and seasonal differences were found for body mass (Fig. 1a). 2) Sex difference was only observed for wet mass of heart, especially, males showed higher wet mass of heart than females in spring but lower in summer (Fig. 1b). Additionally, there was no sex difference in the other internal organ wet masses. 3) The wet mass of liver in summer and winter was higher than that in spring and autumn (Fig. 2a). The wet mass of spleen in autumn was higher than that in spring and winter (Fig. 2b) .The wet mass of lung showed the highest value in spring and the lowest value in summer (Fig. 2c). The wet mass of kidney in spring and winter was higher than in summer (Fig. 2d). The wet mass of heart in autumn was greater than in summer and winter for males. However, it is higher in summer and autumn than in winter for females (Fig. 1b). The wet masses of testis (Fig. 2e) and uterus (Fig. 2f) showed the highest values in spring and the lowest values in autumn. 4) The wet masses of liver, lung and kidney showed higher values in pregnant individuals than non-pregnant ones (Table 1). These results indicate that only the wet mass of heart has significant sex difference and the difference changes with seasons. All of the internal organs exhibit some flexibility characters with the change of seasons, whereas the body mass holds stabilization. The reason may be related to the reproductive status of male and female squirrels and the diversities of season environments.