Abstract: The toxicities of phoxim to Paratrioza sinica in different stages were tested, and the kinetic constants of carboxylesterase (CarE) and the inhibition of phoxim on CarE were also studied. The results indicated that the median lethal concentration (LC50) of phoxim to P. sinica increased with the increase of the nymph stage, but it marginally decreased during the adult stage. The LC50 of phoxim to the first instar nymph was 0.047 mg/L. The LC50 to the fifth instar nymph (0.995 mg/L) was 21.2 times higher than that of the first instar nymph. The Km and Vmax values of CarE in P. sinica increased as the nymph grew older. The median inhibitory concentrations (I50) of phoxim to CarE was also increased with the growth of nymph. The results showed that the toxicities of phoxim to P. sinica were closely related to the activities of CarE in different stages. To achieve good control effect of P. sinica, phoxim should be applied to the small nymph.