The geometry regulating laws of forward and rear bypass injectors of variable cycle engine were analyzed. A mathematic model capable of calculating the variable cycle engine steady and transition performance was established. It considered the local total pressure loss caused by sudden area change at forward bypass injector and mode selector valve. The impacts of geometry regulating laws of forward and rear bypass injectors on variable cycle engine performance and stability were simulated with single bypass mode

double bypass mode and mode transitioning

and the simulation results were compared with NASA test data. The results indicate the opening of forward bypass injector's inner bypass area can decrease recirculation margin and increase fan and core drive fan stage surge margin in single bypass mode. During the late period of transforming from single into double bypass mode

the reduction of fan pressure ratio will diminish by closing rear bypass injector's outer bypass area. It's suggested that the forward bypass injector's inner bypass area should be changed

and the total area should be kept unchanged

while the rear bypass injector's outer bypass area should be changed

and the inner bypass area should be kept unchanged.