【Aim】 The predation of Plutella xylostella by Sycanus bifidus in different insect states was investigated to provide a theoretical basis for the biological control of P. xylostella. 【Method】 The predation functional response, search effect, and interference response of old nymphs and adults of S. bifidus to the 3rd and 4th instar larvae of P. xylostella were studied under laboratory conditions. 【Result】 The functional responses of the 3rd, 4th, and 5th instar nymphs and adults of S. bifidus towards the 3rd and 4th instar larvae of P. xylostella conformed to the Holling II type disk equation, indicating that, the degree of predation increased with the increase in prey density. The female adults and 5th instar nymphs of S. bifidus showed higher daily maximum predation rates towards the 4th instar larvae of P. xylostella, i.e., 93.63 and 58.34, respectively, compared with those towards the 3rd instar larvae of P. xylostella. Compared with nymphs, the predation efficiencies (a/T) of the female adults of S. bifidus towards the 3rd and 4th instar larvae of P. xylostella were better, i.e., 60.98 and 103.65, respectively. The degree of predation of S. bifidus was positively correlated with prey density, whereas the search effect was negatively correlated with prey density, with the highest search effect being observed in female adults. With an increase in the densities of S. bifidus and its prey, the intraspecific interference increased, and the predation rate decreased. 【Conclusion】 The results showed that S. bifidus had good potential for the prevention and control of P. xylostella and could be used for pest management practices targeting P. xylostella.