Because of its simple structure, high power density and good reliability, the Contact-separation mode triboelectric nanogenerator (CS-TENG) shows a good development potential in the field of ambient vibration energy harvesting. Considering the contact and collision force between the moving electrode and the fixed electrode, the piecewise-linear motion differential equation of the moving electrode is derived. Combined with the equivalent circuit differential equation of CS-TENG, an electromechanical coupling model considering the nonlinear variation of electrode spacing is established. Harmonic balance method and arc length extension technique are used to solve the dynamic output characteristics of CS-TENG and judge the stability of the system. Both numerical integration and dynamic tests are applied for verifying the accuracy of harmonic balance results. The effects of excitation amplitude, contact stiffness and structural damping parameters on the dynamic output characteristics of the system are discussed. The results are useful for the structural optimization design and practical development of CS-TENG.