Abstract: In order to clarify the characteristics of predatory behavior of Arma chinensis on the larvae of Pieris rapae and evaluate the predatory capacity of A. chinensis. The predatory behavior and the predatory capacity of adults and nymphs of A. chinensis to the larvae of P. rapae were studied by means of indoor observation combined with the method of HollingⅡ disc equation fitting. The process of predating could be divided into 4 kind of behaviors: searching, piercing-sucking, waiting and feeding. A. chinensis first fed on the larvae of P. rapae by piercing-sucking through its chest, or by piercing through multiple positions on its body. After the larvae of P. rapae were controlled, multiple A. chinensis would gather to feed. The predatory functional response of 4th, 5th instar nymphs and adults of A. chinensis to 4th instar larvae of P. rapae was agreed with the HollingⅡ disc equation, the predation increased with the increase of prey density, and the searching efficiency decreased with the increase of prey density; the sequence of attacking efficiency was as followed: 4th instar nymphs（0.927）, 5th instar nymphs（0.906）, female adults（0.894）and male adults（0.877）; the maximum daily predation of adults A. chinensis was 7.645 individuals, but 5th instar nymphs and male adults of A. chinensis was equivalent, the searching efficiency of 5th instar nymphs and male adults of A. chinensis was similar. This study showed that both adults and 5th instar nymphs of A. chinensis had better predation ability on the larvae of P. rapae, and could be used as a means of biocontrol against P. rapae. therefore, a large number of A. chinensis can be released in the early stage of P. rapae occurrence to reduce the population of P. rapae and minimize their harm to cruciferous vegetables.