<p>Water quality evaluation (WQE) is very important for environment protection and management. There are many WQE methods, and the results from the different methods may differ with one another. However, it is unknown which is more reasonable method. The purpose of this paper is to develop a reasonable approach to determine the final WQ grades/ranks (WQGs) by integrating the WQGs from the individual methods. We develop a novel integrated WQE approach combined with multiple WQE methods based on weighted membership degree. It consists of the following steps: determining the WQGs of the evaluation objects using multiple WQE methods, determining priority weights of the individual methods, calculating weighted membership degrees that the object are evaluated to every grades/ranks, and calculating final WQG indices of the objects. It was applied to two application cases. The first case evaluated the final eutrophication grades of 12 lakes by integrating the grades from scoring index method, variable fuzzy sets method, hybrid fuzzy and optimal model, and neural network method. The second case evaluated the final WQ ranks of 29 wells by integrating the ranks from WQI method, compromise programing method with <i>p</i> = 1 and <i>p</i> = 2, ordered weighted averaging method, and TOPSIS method. In two cases, the proposed approach had the maximum correlation and minimum deviation from the other methods. It illustrated the reasonability of the proposed approach. It could be also used to integrate the environmental quality evaluation results obtained from various evaluation methods in many practical environmental quality evaluation problems.</p>