The invention discloses a calculation method for evaluating the influence of mode conversion on the stability margin of a turbojet/turbofan variable cycle engine. Firstly, a calculation model used forevaluating the inlet and outlet flow dynamic change of the adjusting structure in the mode conversion process of the variable-cycle engine is provided; and then when the mode conversion of the variable-cycle engine is analyzed, the functional relationship between the angle of a mode conversion valve, the opening degree of the front variable-area duct ejector, the opening degree of the rear variable-area duct ejector and the flow is calculated. According to the method, the calculation of the engine stability margin change when the turbojet/turbofan variable-cycle engine dynamically changes theworking modes of single-duct turbojet cycle, double-outer-duct large-duct-ratio turbofan cycle, single-outer-duct small-duct-ratio turbofan cycle and the like is realized.