The optimum ecological water level (OWL) guarantees stable and healthy operation of the lake in the long term by meeting various needs of the lake ecosystem. Previous studies on OWL mainly focused on restoring natural water level regime. However, water-carrying lakes have complicated water lever regimes resulted from flood control, water supply, shipping and other functions. Meanwhile, the water quality was further deteriorated due to rapid economic development. Consequently, the previous methods could not meet all the ecological needs of water-carrying lakes. Basing on IHA (indicators of hydrological alteration)-RVA (range of variability approach) method and relationship between water quality and water level, this paper proposed a method for identifying of the threshold of monthly OWL and determining the OWL variation of water-carrying lake. Taking Lake Hongze as an example, the results indicate that: 1) The lake regulation cycle can be divided into flat period (January-April), discharge period (May-June), early storage period (July-September) and late storage period (October-December) from the angle of hydrological regime and pollution into the lake. 2) There's a strong correlation between water quality and water level in each period as water quality improves with water level rising during the early storage period and deteriorates in other periods (Pearson coefficient was -0.77). 3) Due to the competition between the lake's self-purification capacity and pollutant retention ratio, the monthly OWL threshold from January to December is: 12.92-12.99, 12.79-12.99, 12.84-12.99, 12.86-12.99, 12.71-12.89, 12.39-12.63, 11.97-12.93, 12.50-13.07, 12.65-13.26, 12.90-13.04, 12.90-13.04, 12.90-13.04 m, respectively. The range of OWL modified by the relationship between water level and water quality is reduced by 73.4 % compared with the results of IHA-RVA method except in the early storage period. In general, the OWL designed by methods in this paper can reflect the requirements of lake ecosystem for water quantity and water quality to a certain extent, and provide scientific basis for the ecological regulation and water resource management of Lake Hongze and other water-carrying lakes.