Taking the discontinuous capacitor voltage mode Cuk power factor correction (PFC) converter as an example, the slow scale bifurcation phenomenon when the system load increases is described. This paper analyzes the characteristics and causes of these phenomena, deduces the harmonic balance solution of the system according to power balance, and analyzes the influence of load resistance on the system stability based on Floquet multiplier method. The simulation results show that with the increase of the load resistance, the system will enter the period doubling bifurcation and then enter the chaos state, circuit light load easy bifurcation. This kind of slow scale bifurcation phenomenon and related analysis have been verified experimentally.