Extreme precipitation is one of the important meteorological factors affecting water quality. With global warming, the frequency and magnitude of extreme precipitation events have increased significantly, which greatly influenced the hydrochemistry and water environment in lakes. Lake Baiyangdian is an important wetland and ecological area in North China. In order to identify the hydrochemical characteristics and water quality variations in the main area of Lake Baiyangdian under an extreme precipitation event, the Piper diagram, Gibbs diagram, and multivariate statistical methods were used to elucidate the hydrochemical characteristics, hydrogen and oxygen stable isotope characteristics, and spatial variations of water quality in this study. Accordingly, the effects of the extreme precipitation event on the hydrochemistry and water quality were detected in the main area of Lake Baiyangdian. The results showed that (1) Lake water is weakly alkaline, and the hydrochemistry type is Ca-HCO₃·SO₄ under the extreme precipitation event. The extreme precipitation weakened the effects of evapotranspiration and human activities on the chemical compositions of the lake water. The extreme precipitation also led to the large changes in pH, electrical conductivity and total dissolved solid values in the lake water. (2) The hydrogen and oxygen isotopic values (δ²H and δ¹⁸O) respectively ranged from -60.86‰ to -35.01‰ and from -8.84‰ to -3.45‰, showing significant negative correlations with water depths. Although the hydrogen and oxygen stable isotopes of the lake water were depleted by the extreme precipitation, the extreme precipitation did not affect their spatial distribution. The relationship between δ²H and δ¹⁸O values is more strongly influenced by the correlation between δ²H and δ¹⁸O values in precipitation itself. Additionally, the heavy precipitation caused a sudden increase in the water depth of the main area of the lake, which weakened the influence of evaporative fractionation on the lake water. (3) The extreme precipitation raised the diffusion rate of non-point source pollution and increased the concentrations of ammonia nitrogen, nitrate nitrogen and total phosphorus in the lake water. The ammonia nitrogen, in precipitation may also increase the ammonia nitrogen concentration in lake water. Additionally, releasing water from reservoirs located in the upper reaches of the lake in response to extreme precipitation improved the water quality in the northeastern area of the lake. This study provides a scientific basis for the ecological restoration of Lake Baiyangdian and the ecosystem management of Xiong'an New Area.