In order to study the overall scheme design of the medium-small core engine-derived engine, based on the co-working constraints of the core engine’s components and the matching mechanism of high and low pressure system, the influence of the core engine design points on the characteristic parameters of core engine with non-variable geometric area and variable geometric area is studied, the overall performance and size-weight calculation model of the core engine derived engine is established, and the model is used to design the overall scheme design of a big bypass ratio turbofan engine derived from an advanced medium-small core engine. The results show that the mechanical load and thermal load of the core engine can not be simultaneously exerted when the geometrical area of the core engine is not variable. Only when the geometrical area of the core engine is variable, the mechanical load and thermal load of the core engine can be simultaneously exerted. When the pressure ratio of the low-pressure system is 1.8, compared to the core engine with non-variable geometric area, the core engine with variable geometric area increases 16.4% in mass flow, 14.1% in thrust and decreases 1.4% in specific fuel consumption.