Abstract: To investigate whether phosphorus improve tolerance to thrips effectively, two alfalfa varieties were chosen for the study, Gannong No.9 (resistant to thrips) and Gannong No.3 (susceptible to thrips). The damage index, phosphorus content in plants and agronomic traits were evaluated and measured at different phosphorus (P₂O₅) levels (0, 6, 12, 18 g/m²) treatment during the thrips damaging peak in field. The results showed that the damage index of Gannong No.3 and Gannong No.9 decreased with the phosphorus levels increasing, in addition, phosphorus content of plants increased, and yield, height and leaf water content increased significantly, the stem-leaf ratio reduced. Correlation analysis indicated that the phosphorus content of plants and leaf water content correlated negative significantly with the damage index of alfalfa, but the phosphorus content of plants had no correlation with the damage index in second cutting,while the stem-leaf ratio correlated positively with the damage index. The damage index of Gannong No.3 were higher than Gannong No.9 at the same phosphorus level, but no obvious difference with Gannong No.9 at the phosphorus (P₂O₅) levels (12,18 g/m²) treatment in second cutting. Gannong No.3 damage index with phosphorus treatment were lower than Gannong No.9’s without phosphorus application. The phosphorus can enhance the resistance of alfalfa to thrips effectively by improving the growth performance. Therefore, the phosphorus management was an effective measure on enhancing the tolerance of alfalfa to thrips in field. The most economic fertilizer was 12(P₂O₅) g/m² in this experiment.