A biasing voltage generating circuit for generating a required reverse biasing voltage of an avalanche photodiode (APD) includes: a boost power converter configured to operably convert an input voltage into a higher output voltage according to a feedback signal and a reference signal, and to apply the output voltage to be a reverse biasing voltage of the APD; a reference signal generating circuit configured to operably generate the reference signal; and a control circuit. The control circuit includes: a signal sensing circuit configured to operably generate a sensed signal corresponding to an output current of the APD; an analog-to-digital converter (ADC) configured to operably convert the sensed signal into a digital signal; and a processing circuit configured to operably adjust the feedback signal or the reference signal according to the digital signal to thereby control the boost power converter to adjust the output voltage.