In order to reduce the probability of fish injury and improve the fish passing capability for water turbines, an evaluation method for fish passing capability was established based on the fish damage mechanism. Combined with a three-dimensional hydrodynamic distribution characteristic of the turbine passage and the hydrodynamic damage threshold of the fish, the evaluation method was proposed for fish passing ability. The key operation parameters for Kaplan turbines were discussed based on the study case in which the juvenile of the four major Chinese carps in the Yangtze River passing through the ZZ500 turbine passage in the Gezhouba Hydropower Station. The results show that the operating head, flow rate, guide vane opening, blade angle and power output of the Kaplan turbine all have certain influence on the fish passing capability, and the key influence parameters are operating head and guide vane opening. When the turbine operates under the conditions of low head, large flow, large output and high efficiency, the probability of fish injury caused by the spatial hydrodynamic characteristics of the flow passage is small, and the fish passing capacity of the turbine passage is the best.