To observe the influence of stocking density on the growth and survival rate of P. clarkill and to investigate the suitable stocking density of P. clarkill at the size, Procambarus clarkill Girard (initial weight of 2.4 g±0.2 g）were reared at different stocking densities of 12, 20, 32, 40, 52 and 60 individuals·m^-2 for two months. The relative growth rate, the survival rate and the limb loss rate were calculated. The activities of ACP, LZM, SOD and CAT in serum and the phagocytic activity of blood cells were determined after 60 days feeding period in order to observe the influence of stocking density on the growth and survival rate of P. clarkill, to investigate the probable reasons and confirm the fitting stocking density of P. clarkill at the size. Results showed that the relative growth rate, the survival rate and the activities of ACP and LZM in the serum showed a declining trend and the limb loss rate presented a gradually rising trend with the increasing stocking density. The activities of SOD and CAT in the serum were first increased and then decreased with the increasing stocking density. We concluded that the decreased growth rate, reduced survival rate, and increased mortality rate as the increasing stocking density of P. clarkill might be caused by the increased competition for the energy resources and territorial space among the individuals. The free radical scavenging ability was decreased, and the "free radical homeostasis in dynamic" was broken, which resulted in the antioxidant system disorder, the organism oxidative damage, cellular and humoral immunity declined in the body of P. clarkill. Twenty individuals·m^-2 might be a suitable stocking density of P. clarkill at the size of 2.4 g±0.2 g, considering the growth rate, the survival rate, and adequately utilized water resource.