Helical gear transmission is one of the widely used mechanical driving. During the gear meshing process

impact will arise. The new non-linear simulation function （SOL600） of MSC Nastran is used to build the 3-D contact meshing model for helical gear and the impact excitation is calculated. Modal transient response method is used to get the dynamic characters of helical gear on impact excitation . Appropriate reference is provided for optimization of helical transmission system.