In view of removing temporary supports from the large span steel structural roof, a new unloading method which was called constant-force jack unloading method was proposed. The constant jack with COMBIN37 element of ANSYS was simulated. Then a classic numerical analysis model was constructed, and was analyzed respectively by support displacement method, the equivalent rod end displacement method, jack element method, constraint equation method and constant-force jack unloading method. The analysis results show that compared with the traditional displacement-control method, the constant-force jack unloading method can effectively control the force change of the jack. The internal force and deflection of the structure were more efficiently controlled. The constant-force jack unloading method is especially applicable to the structure with large stiffness.