A mathematical model of turbofan engine considering ambiance humidity is established, according to certain control law, performance humidity modify coefficients of turbofan engine are calculated, the results show that humidity modify coefficients have the same trend with foreign mature turbofan, turbojet engines' modify coefficients, but the values are not unequal because of different engine types, computation arithmetics and control laws, all of this can provide theoretical data supports to expanding ambiance humidity research of turbofan engine.