In this paper, the cycle analysis and installation characteristics of the engine are analyzed, and the variable cycle mechanical structure is designed. The intermediate series fan variable cycle engine moves the fan of the traditional large bypass ratio turbofan engine back to the low and high-pressure compressor connection, so it becomes the intermediate series fan. The fan adopts CDFS (core engine fan) technology and has the characteristics of high speed and a large boost ratio. In front of the fan, the auxiliary air intake of the external culvert is designed, and the intake regulating device is installed to control the airflow of the external culvert and adjust the bypass ratio. The adjustment structure refers to the camera aperture adjustment mechanism and has the characteristics of a continuous adjustable opening and a stable intermediate process. The movement direction of blade adjustment is perpendicular to the direction of incoming flow, and the influence of incoming flow impact on blade movement can be minimized in the adjustment process. It is driven by a hydraulic actuator cylinder and has the advantages of a simple structure and easy realization. A mixed turbofan variable-cycle engine with an intermediate series fan can provide thrust for civil airliners with a flight speed of Mach 0–3 and has good economic performance. The proposed scheme has certain reference significance and value for the development of the next-generation supersonic airliner and the development of a military variable cycle engine.