Abstract: This paper deals with the impact of stocking silver carps into fish farming reservoirs on the carrying capacity of the reservoirs for feeding-cage-culture of common carps. 30 enclosures were used in the experiment. Every enclosure was 5 m in deep and 14.3m3 in capacity. 8 physical and chemical factors and 5 biological factors were examined periodically during the experiment, and results of fish rearing of every ericlosure were analysed in the end. Results indicated that stocking of silver carps improved water quality obviously in respects of DO, COD and TP, and remarkably diminished phytoplankton, zooplankton and planktonic bacteria in the water. Thereby it improved the results of carp rearing distinctly in terms of net production, growth rate, and food efficiency, and increased the totel fish production and total carrying capacity by 35%. As for the optimum stocking ratio (by weight) of common carp to silver carp, it was found out in the experiment that 3:1 is superior to 2:1.