Abstract: Rivers serve as a link between the land and the ocean, transporting a large amount of terrigenous substances from the land to the sea. The instability of the concentration of base cations in rivers has a significant impact on the water environment of the river basin. Investigating distribution characteristics of base cations and their influencing factors during tidal cycles in the tidal reaches of rivers is of great significance for understanding the ecological environment of rivers flowing into the sea. The 117-km tidal reach of the lower Minjiang River (divided into upper, middle, and lower segments) was selected as the study area, and the surface water at 11 sampling points was collected during the flood tide and ebb tide processes on a spring tide day. The concentrations of base cations such as K⁺, Ca²⁺, Na⁺ and Mg²⁺ in the water and their relationships with environmental factors were further analyzed. The results show that the mass concentrations of K⁺, Ca²⁺, Na⁺ and Mg²⁺ cations in the water are 2.28-448.18 mg/L, 7.34-257.89 mg/L, 4.54-7 898.50 mg/L and 1.39-892.44 mg/L, respectively. There are relatively small changes in measured base cations in the upper and middle reaches of the tidal reach, but the lower reach exhibits a significant increase. In the upper and middle reaches of the tidal reach, the proportions of base cations concentrations such as K⁺, Ca²⁺, Na⁺ and Mg²⁺ in the water are 12.26%-15.31%, 32.66%-47.63%, 29.03%-45.46% and 8.61%-9.63%, respectively. In the lower reach, the proportions of K⁺ and Ca²⁺ cations in the water are significantly decreased, and the proportion of Na⁺ cations is dramatically increased, while the proportion of Mg²⁺ cations is relatively stable. Compared with the flood tide, the ebb tide process decreases the concentrations of K⁺, Ca²⁺, Na⁺ and Mg²⁺ cations to a certain extent. Especially in the lower reach, the decline ratios are about 25.19%-69.48%, 19.18%-53.75%, 22.61%-87.13% and 21.47%-81.67%, respectively. During the flood tide and ebb tide processes, the concentration of base cations is significantly positively correlated with pH, salinity and the concentration of suspended substances. During the ebb tide process, the concentration of base cations also has an obvious positive correlation with the water temperature. Comprehensive analysis further shows that tides mainly affect the concentrations and distribution of base cations by changing the mixing of fresh water and salt water, and the upstream movement of seawater caused by the flood tide has a more obvious impact on this process. These results not only reflect the hydrochemical characteristics of the river basin under the action of natural processes and human activities, but also help to understand the complex material migration process between the land and the sea. Nevertheless, this study only sampled on a spring tide day with strong tidal effects, lacking a comprehensive exploration of the concentration and distribution of base ions on neap tides. Overall, our results provide a scientific basis for the ecological protection and sustainable development of the river basin.