Abstract: The wireless power transfer（WPT） system with the LCC-S compensation（LCC compensation on the primary side and LC series compensation on the secondary side） has the advantages of constant output current on the transmitting side and the high tolerance to the misalignment of the coupling mechanism, which has been widely used in recent years. However, the inherent nonlinear characteristic of the WPT system may lead to system instability. Therefore, this paper takes the LCC-S compensated WPT system as the research object to analyze the instability phenomenon of the system. Firstly, the harmonic state space（HSS）theory is introduced to the modeling of the WPT system, and the method of transforming the state-space equation in the time domain into the HSS equation is given. This modeling method could achieve high accuracy since it takes into account the mutual coupling effect of various harmonics in the system. Secondly, the Pade approximation is used to approximate the time delay link,and the small-signal model of the system in the closed-loop control mode is derived based on the HSS theory. Thirdly, the influence rule of controller parameters and hardware parameters on the stability of the WPT system is studied through the distribution of eigenvalues. Finally, the simulation and experimental results are provided to verify the accuracy of the proposed mathematical model and the effectiveness of the theoretical analysis.