The hybrid system of Brayton cycle and ambient pressure gas turbine cycle（APGC）was described and the results of performance study was presented.The performance of the system was calculated by using the commercial process simulation package Aspen Plus.The results revealed that the optimum performance of the hybrid system could be obtained by changing upper cycle pressure ratio and bottom cycle expansion pressure.The expert factor analytic method，which is significant in theoretical design and practical application of a new proposed cycle，was also presented to provide the design pressure ratio of the combined cycle according to the usage of system.