In this paper, we investigate the intermediate-frequency oscillation in a SEPIC power-factor-correction (PFC) converter under one-cycle control. The converter operates in continuous conduction mode (CCM). A systematic method is proposed to analyze the bifurcation behavior and explain the inherent physical mechanism of the intermediate-frequency oscillation. Based on the nonlinear averaged model, the approximate analytical expressions of the nominal periodic equilibrium state are calculated with the help of Galerkin approach. Then, the stability of the system is judged by the Floquet theory and the Floquet multiplier movement of the monodromy matrix is analyzed to reveal the underlying mechanism of the intermediate-frequency oscillation behavior. In addition, Floquet multiplier sensitivity is proposed to facilitate the selection of key parameters with respect to system stability so as to guide the optimal design of the system. Finally, PSpice circuit experiments are performed to verify the above theoretical and numerical ones.