With consideration of the fact that bias voltage for Si-APD changes in different ambient temperature and the corresponding electric network requires low ripple voltage, a kind of bias circuit is built based on Royer oscillator and MCU. The circuit is made up of four key parts (including their peripheral networks): synchronous Buck-converter controller BL8032, supplying input power source for Royer oscillator; DAC MS5221M, serving as the direct adjustment of the input power source for Royer oscillator; ADC AD7890, with the OPA AD8606 as an buffer, used for sampling output voltage of Royer oscillator; and an MCU STM32F103TBU6, working as the main computing and timing sequence controller. This bias circuit not only provides low ripple, low noise and low power consumption, but also meets requirement of wide input voltage of 9~36 VDC and safety electric isolation. It works well at ambient temperature from -40 ℃ to 70 ℃.