Abstract: In order to explore the spatial distribution pattern of thrips on alfalfa （Medicago sativa）, aggregation indexes and Iwao regression methods were used in this study. The results showed that the distribution of thrips on alfalfa behaved as the clustered colonies and the aggregation intensity decreased with the increasing of cutting times. The aggregation was affected by environmental factors and thrips habits. Based on the parameters fitted by the Iwao model, the equation of optimal sampling number was proposed.