Abstract: To delay avermectin photo-degradation, the paper studied the effects of different concentrations of antioxidant, terliary butyl hydroqulnon (TBHQ), on avermectin photo-degradation. The toxicity of avermectin and TBHQ mixture on small brown planthopper (SBPH) Laodelphax striatellus (Fallén) was also determined via rice-stem-dipping method. The results indicate that avermectin photo-degradation fits into the equation for first-order chemical reaction kinetics. Photolysis rate of avermectin photo-degradation under sunlight is slower than that under UV light. When TBHQ and avermectin ratio is 1︰1, photolysis rate is lowest for avermectin photo-degradation, and its half-life is 11.81 h. This is 167.19% increase compared with treatment without TBHQ. Laboratory bioassay results demonstrate that the synergistic effect of TBHQ and avermectin mixture on SBPH is significantly strong. After 3 h exposure to sunlight and UV light, the toxicity of avermectin and TBHQ mixture on SBPH increases significantly compared to that without TBHQ. The synergism ratio is 2.00 and 2.63, respectively.