Difficulties in obtaining component characteristics in the sub-idle state of rotor constrain the simulation capabilities of ground and windmill start-up processes for turbofan engines. This paper proposes a backbone feature method based on conventional characteristics parameters to derive the full-state characteristics of fan. The application of the fan’s full-state characteristics in component-level model of turbofan engine enables zero-speed iterative simulation for ground start-up process and windmill simulation for windmill start-up process， thereby improving the simulation capability of sub-idle state during turbofan engine start-up.