Abstract: In order to understand the behavior response of the flea beetle, Agasicles hygrophila, under different sex ratios, and to clarify the mechanism of adults encountering and mating behavior response, the observation and analysis of adults behavior under three sex ratio conditions, which were one female and one male, one female and one female, one male and one male, were carried out by laboratory observation in combination with automatic record and analysis through animal behavior observing analysis system (CASO-L Behavior Analysis) within a 9 cm arena. The results showed that the encountering behavior was composed of avoidance and fight between two female adults or two male adults, and avoidance was highly significant than fight. In the process of a couple of flea beetles encountering, the frequency of copulation was relatively small while the avoidance was the major behavior, and they might mate after several meetings. A. hygrophila had a distinct mating circadian rhythm. Newly emerged adults do not mate at the first to the second day, and began to mate at the third day. The probability of mating activity was low between 00:00 and 10:00, and the distinct mating peaks occurred at 13:00-15:00. Overall, A. hygrophila adult male and female individuals existing at the same time is appropriate to the preservation, and could reduce the fighting behavior between individuals. Indoor adult paired spawning could be selected within 3 days after the emergence, and 13:00-15:00 was the appropriate time frame.