When the tooth lead modification was performed on a helical gear by form grinding method, there are existing the phenomenon of modification twist. In order to improve the twist error of tooth lead modification, a mathematical model of form grinding is established for tooth lead modification to solve the simultaneous contact line between the grinding wheel and gear surface based on gear engagement theory. By analyzing the influence of grinding wheel radius and grinding wheel setting angle on tooth lead modification contact line, the three methods were proposed based on optimizing the grinding wheel profile, the wheel setting angle, both the wheel profile and setting angle. Finally, as an example of helical crowning gear, the proposed methods are validated through the model and experiment separately，which can effectively improve the modification error in form grinding.