In order to study the stability of a grid-connected direct-drive permanent magnet synchronous generator (PMSG) wind turbine systems, this paper presents the modal analysis of a PMSG wind turbine system. A PMSG model suitable for small signal stability analysis is presented. The modal properties of a grid-connected PMSG wind turbine system are studied. Then the comprehensive impacts of the shaft model, shaft parameters, operation points and lengths of the transmission line on the modal characteristic of the system are investigated by the eigenvalue analysis method. Meanwhile, the corresponding nonlinear model is set up in the PSCAD/EMTDC, and the output power curve is analyzed by Prony method to validate the modal analysis results. The results show the importance of shaft two-mass model and the relationship between the stability of PMSG system and the system parameters from the angle of modal analysis. It offers a better understanding about the essence of the stability of grid-connected PMSG wind turbine system.