Abstract: Excessive application of phosphate fertilizers in agriculture leads to the loss of unused phosphorus, which poses a serious threat to water quality, identifying key source areas of phosphorus loss in agricultural fields and their influencing factors is crucial for effective pollution management. While the Phosphorus Index Model (PIM) has been widely applied and developed for evaluating phosphorus loss risk and identifying key source areas in small watersheds, research on phosphorus loss risk assessment in wheat-corn rotation fields at different scales remains limited. This study focuses on the core areas of the Baiyangdian Basin, specifically Baoding, Dingzhou, and Xiong'an New Area. It constructs a phosphorus index evaluation system using phosphorus surplus from a wheat-corn rotation system and available soil phosphorus as source factors, along with surface runoff, distance to rivers, and distance to lakes as transport factors. The research examines phosphorus loss risks and key influencing factors in wheat-corn rotation fields over different years (2015, 2017, and 2019) and scales (sources, transport to rivers, and transport to the lake). The results show: that during the study period, key source areas for sources, and phosphorus loss were primarily concentrated in the northeastern and southwestern agricultural regions of the study area, with a 2.7% reduction in extremely high-risk zones from 2015 to 2019. Key risk areas for phosphorus transport to rivers were more prevalent along both sides of the rivers, with a 0.95% decrease in extremely high-risk zones during the same period. The critical source areas for phosphorus transport to the lake lakes were primarily located in the southwestern and northeastern counties surrounding the target water body, showing a 3% reduction in extremely high-risk zones. Key influencing factors for phosphorus transport to the lake included agricultural phosphorus surplus and transport factors such as surface runoff and distance to the lake, both of which were significantly correlated with phosphorus loss risk (P < 0.01). Agricultural phosphorus surplus emerged as the main influencing factor, with an average correlation of 0.72(P<0.01), primarily sourced from chemical fertilizers, averaging 69.32%. These findings provide scientific reference for assessing phosphorus loss risk in Baiyangdian and offer guidance for effectively managing agricultural non-point source pollution.