The hunting frequency is an inherent characteristic parameter in the wheel-rail system and can affect the modal behavior of the vehicle system due to its continuous variation with running speeds, which in turn dominates some structural resonance related-dynamics. This study focuses on the variation mechanism of the hunting frequency in the railway wheelset system. The variables in the wheelset system are controlled to study their effects on hunting frequency variation under speed parametric excitation. The dynamical hunting frequency is always lower than the kinematical hunting frequency due to the drag effect of the inertia forces. The saturation mechanism of hunting frequency is revealed and the limit value is analytically expressed. The effects of nonlinearities, including the wheel flange force and the creep forces, on the fundamental hunting frequency and the high-order harmonic frequencies are investigated. The results indicate that the nonlinearities should be considered for the precise calculation of hunting frequency.