【Objective】Intensive vegetable fields have been the priority control projects for phosphorus (P) loss in farming due to massive fertilization and flood irrigation, accounting for the highest proportion of P pollution from farmlands. Therefore, (i) quantifying the assessment of the annual P loss of vegetable fields, (ii) clarifying the reasonable threshold range of phosphate fertilization for vegetables, and (iii) controlling P migration and loss from vegetable fields through the source are of great significance for effectively reducing the environmental pressure caused by agricultural non-point source pollution. 【Method】A plot experiment in the Tai Lake Basin with conventional fertilization and phosphate fertilization reduction treatments was conducted in a perennial and open vegetable field. The one-year vegetable rotation experiments focused on clarifying the environmental threshold of P and annual P runoff loss characteristics. There were five treatments, conventional fertilization (CK), reduction of 20% (P-20), 30% (P-30), 50% (P-50), and 100% (P-100)based on conventional P fertilization. 【Result】 The environmental threshold of soil P in the tested vegetable fields was 78.9 mg·kg^–1. During the whole experimental period, Olsen-P content in the soil surface of different treatments exceeded the leaching threshold. The results of annual total P (TP) runoff loss concentration in vegetable fields showed that with the decrease of phosphate fertilization input, the annual loss concentration of TP decreased. Also, dissolved P (DP) was the dominant form of P species, accounting for 50.1%～63.1%. Runoff loss loads of P showed seasonal characteristics, with the amount of P loss in a summer-autumn season (1.93～3.26 kg·hm^–2), accounting for 59.2%～63.2% of the annual P loss flux. Based on the structural equation modeling (SEM), the amount of phosphate fertilizer had a direct and extremely significant effect on TP loss concentration. Furthermore, TP loss concentration had a positive and extremely significant impact on TP runoff loss load with a path coefficient of 0.97. The loss coefficients of P were between 1.36% and 3.33%, and the lowest loss coefficient was in the P-50 treatment. With a decreased amount of P applied, the reduction ratio of P runoff loss increased, and a P loss reduction rate of 41.5% was recorded in the P-100 treatment. Treatments of P-20 and P-30 had no significant yield reduction during a three-successive vegetable growth period in a year compared with CK. 【Conclusion】 Taking environmental risk and economic yield into consideration, it is appropriate to reduce between 20%～30% P fertilizer application based on conventional phosphate fertilizer rate for open vegetable fields in Tai Lake region.