A buck-boost converter working in a buck mode with buck switching cycles, a boost mode with boost switching cycles or a buck-boost mode with buck-boost switching cycles. Each of the buck-boost switching cycles has an AC phase, an AD phase and a BD Phase, and the duty cycle of the AC phase is determined by a reference signal, a feedback signal and an inductor current sense signal, the duty cycle of the AD phase is controlled and maintained at a preset duty threshold, the time period of the buck-boost switching cycle equals the time period of the buck switching cycle and the time period of the boost switching cycle.