The control strategy of a four-switch buck-boost converter with constant current/constant voltage modes was studied. When the input and output voltages were close to each other, buck-boost mode was adopted to realize smooth switching between buck mode, boost mode and buck-boost mode by detecting the duty cycle of the power FETs under different relationship between input and output voltages, thus improving the stability of the system. By designing the maximum value selection circuit, the converter could automatically transit from constant current mode to constant voltage mode in charging application, and the mode transition was smooth and stable. The simulation results show that at 24 V output voltage, when the converter switches from buck mode to buck-boost mode, the output under-shoot voltage is 9.2 mV. When the converter switches from boost mode to buck-boost mode, the output under-shoot voltage is 92 mV. The converter can automatically realize smooth conversion between constant current and constant voltage modes in both buck mode and boost mode.