Aiming at the shortcomings of environmental energy harvesting systems such as high output voltage ripple and efficiency changing with load, an energy harvesting system with high conversion efficiency and low output voltage ripple in a wide load range is proposed. The system was based on the optimized conduction time (OOT) control method to regulate the output ripple, which solved the problem of a large ripple in the case of small load capacitance and light load in the traditional control method. In addition, the adaptive system clock frequency (ACF) control method could improve the efficiency of traditional methods in light loads, and maintain high efficiency in a wide range of loads. A 180 nm CMOS technology was used to verify the design of the energy harvesting system. The simulation results show that the peak efficiency of the designed energy harvesting system is 89.75%, and the lowest efficiency is 83.75% in the range of 1 mA load current, which is more than 7 percentage points higher than that of similar systems. The ripple value decreases from 177.96 mV to 23.56 mV at 0.2 μF load capacitance.