This paper studied the relationship between the stiffness and the stability of current collection for high speed train. Based on the traditional study, the differential equation of catenary pantograph system was established, which took into account the high order term of the stiffness Fourier expansion. Using the perturbation method, the stability of the system was analyzed under small damping. Under the assumption of the second order form of the displacement response of the system, the stability boundary was obtained. The effects of damping effect and other parameters on the stability boundary were discussed. Moreover, the proposed solution was compared with the classical solution in references, which indicates that the proposed solution has a more higher accuracy.