[Aim] Avermectin and β-cypermethrin with different mechanism have been used to control Tirathaba rufivena, but the synergism between these two pesticides was uncertain.[Method] The bioactivity and synergism of avermectin and β-cypermethrin on T. rufivena were studied under laboratory conditions using bio-assay methods.[Result] Avermectin and β-cypermethrin showed stomach toxicity on T. rufivena with LC₅₀ of 22.24 mg·L^-1 and 18.18 mg·L^-1. The mixture of avermectin and β-cypermethrin with a mass concentration ratio of 4:5 and 9:5 showed a co-toxicity coefficient (CTC) of 93.78 and 107.78, respectively. The significant synergism effect was shown in the mixture with a mass concentration ratio of 5:1 with the CTC of 259.07. The mixture with a mass concentration ratio of 3:10 showed an antagonistic effect with the CTC of 76.72.[Conclusion] Different effects exist at different proportions between avermectin and β-cypermethrin. The synergism was the strongest at a ratio of 5:1.