This paper presents a Charge-Pump-based SIMO (CPSIMO) buck-boost converter with three operation modes. By modifying the power stage topology of conventional SIMO buck-boost converters, the proposed converter exhibits faster transient response and higher efficiency. Moreover, an average-voltage-based mode selector is introduced to implement the smooth transition between three modes. The proposed converter is designed in TSMC 0.18 um BCD process with a supply voltage of 2.7V-4.2V and has three outputs of 1.8V, 3.3V, 5V. Simulation results show that the proposed converter achieves a peak efficiency of 94.61\% under output power of 2.8W and a load transient response time of 9.9 us with a 1.5W power step.