Two types of water-based preparations of avermectin were developed and the bioactivity and control efficacy to diamondback moth,Plutella xylostella L.,in experiment were carried out.The results from the experiment are as follows: The physical and chemical properties such as surface tension,penetrability,wettability of microemulsion and emulsion in water of avermectin and dimehypo were better than those of emulsifiable concentrate(EC).Among the preparations microemulsion tested was the most stable and viable.The bioassay in laboratory indicated that synergistic ratios of compound preparation of avermectin and dimehypo were higher than those of EC and its synergistic ratio(SR) was 3.24 while LC_(50) value of EC was set as 1.0.Also,they showed significantly more effective than single preparation in plot field and the control efficacy was as high as 89.12% in 7 days after treatment,but no significantly different among single preparations.The water-based preparations of avermectin and dimehypo not only had better control efficacy,but also deferred developmental speed of resistance and reduced pollution of organic compounds in environments.