Abstract: Yuehai Lake is a nationally significant wetland, where water pH influences the stability of the lake ecosystem. To investigate factors influencing Yuehai's elevated summer pH, this study analyzed monthly water quality data from controlled cross-sections in the Yuehai area (2020-2024), daily data from the Yuehai automatic monitoring station, and detailed surveys of water quality during the 2025 summer Yellow River replenishment period. It examined the spatiotemporal distribution of Yuehai's pH and the effects of Yellow River replenishment, evaporative concentration, and biological processes on pH levels. Results indicate: 1) From 2020 to 2024, summer pH levels in Yuehai Lake exceeded winter values, with daily pH readings occasionally surpassing surface water quality standards (pH>9). During the summer of 2025, pH ranged from 8.23 to 9.24, spatially distributed with lower pH near the southeastern inflow point and higher values towards the lake center. 2) Correlation analysis revealed significant positive correlations between pH and dissolved oxygen (DO), and significant negative correlations with Ca²⁺and HCO₃^- . Principal component analysis revealed positive loadings for pH with components representing photosynthetic activity intensity and calcium carbonate (CaCO₃) precipitation-dissolution equilibrium. Yuehai Lake's pH is influenced by Yellow River inflow, evaporative concentration, and biological activity. 3) During summer, the pH of Yellow River inflow water is 8.02. Based on hydrochemical analysis, Yuehai Lake exhibits a Cl^--SO₄^2--Na⁺-Mg²⁺ ion composition, whereas Yellow River inflow water displays an HCO₃^--SO₄^2--Ca²⁺-Mg²⁺ composition, indicating ionic concentration disparities. The Ca²⁺+Mg²⁺/CO₃^2-+HCO₃^-+H₂CO₃^* ratio of the Yellow River inflow exceeds 1. Upon entering the lake, this inflow consumes dissolved inorganic carbon, leading to an increase in lake pH. Gibbs diagrams indicated evaporative concentration in Yuehai Lake, with saturation indices for calcite and dolomite exceeding zero in lake water samples. This demonstrated that evaporative concentration promotes carbonate precipitation, consuming CO₃^2- and HCO₃^- to elevate lake pH. During summer, the partial pressure of carbon dioxide index (logρ(CO₂)) consistently falled below zero while dissolved oxygen saturation (DO%) exceeds 100%, indicating that biological photosynthesis dominates. The lake system absorbed CO₂ and releases O₂, consuming CO₂ within the lake and consequently raising pH. The study explains that the replenishment of water in the Yellow River, evaporation concentration, and biological activities contribute to the high pH values in summer. To alleviate the increase in pH, it is recommended to optimize the water replenishment plan and pay attention to the nutrient status of the lake.