In recent years, wind power is connecting to the grid on a large scale, and the static instability mode of the system is moving from angle instability to voltage instability. Based on the singular perturbation theory and multi time scale reduced order modeling, a simplified model of wind-thermal-bundled system is established for mechanism analysis. According to the criterions of power angle instability and voltage instability, the boundary properties of the steady-state security region of wind-thermal-bundled system are analyzed with the small disturbance method, which is composed of angle instability boundary, hybrid angle-voltage instability boundary and voltage instability boundary. The solution process of breakpoints of three instability modes are proposed. The results are helpful for the dispatch department to analyze the leading factors of instability and take targeted measures to pull the system back to the safe region.