Baiyangdian Lake is an important ecological water body in Xiong’an New Area. Its water environment management has entered a stage of consolidation and improvement, with increasing emphasis on refined management and control. Based on monthly water-quality data collected from 16 monitoring sections in Baiyangdian Lake and seven major inflowing rivers from January 2023 to August 2025, this study analyzed the spatiotemporal variations in water quality and the high-value periods of total phosphorus (TP) in the river–lake system using the water quality index (WQI), nonparametric tests, and high-value frequency identification. The results showed that the mean WQI values of the 16 sections ranged from 57.68 to 71.98, with an overall mean of 65.19, indicating that the comprehensive water quality was generally classified as “medium”; the difference in comprehensive water quality between the river-input units and lake-water units was relatively small. The mean normalized TP score was 22.32, 91.73% of the samples had normalized scores no higher than 50, and the adjusted frequency of TP as the lowest-scoring indicator was 75.48%, indicating that TP is the key limiting factor constraining further improvement of the overall water quality of the Baiyangdian river-lake system during the consolidation and improvement stage. The median TP  concentration in the river-input units was 0.40 mg/L, significantly higher than that in the lake-water units (0.26 mg/L), and remained consistently higher throughout the study period, indicating that the river-input units were spatial units requiring priority attention. High-value frequency identification showed that the TP high-value period occurred from June to October in the lake-water units and from September to November in the river-input units. The high-value periods in the two spatial units overlapped in September and October, but the high-value period in the river-input units occurred later and extended into November. This study provides a reference for phosphorus pollution control in Baiyangdian Lake and for TP monitoring and risk early warning in major inflowing rivers.