Abstract: The optimum reaction condition of carboxylesterase in poplar was studied using the biochemistry methods. The results indicated that the optimum enzyme dilution factor of carboxylesterase was 10, and the optimum substrate concentration was 3×10^-4 mol/L, the optimum pH value was 7.0, and the optimum reaction time was 15 min, and the optimum temperature 30 ℃, for carboxylesterase in poplar. Furthermore, the effect on carboxylesterase from poplar by omethoate, imidacloprid and fipronil was determined. The results showed that carboxylesterase activity of poplar was inhibited by 400,1 600,3 200 and 6 400 mg/mL omethoate, and the inhibition effect of 6 400 mg/mL omethoate was strongest. The carboxylesterase activity of poplar didn’t change, after being treated by 50,100,200,400 and 800 mg/mL imidacloprid for 48 h, and the activity decreased by 400 and 800 mg/mL imidacloprid for 96 h. The carboxylesterase activity of poplar decreased after being treated by 400 mg/mL fipronil for 48 h, and by 800 mg/mL fipronil for 96 h.