Avalanched photodiode(APD) is useful in scanning laser imaging system due to their high sensitivity and fast responsibility. However, the optimal bias voltage of APD circuits is not fixed because of the changing environment conditions and temperature. It is proved that keeping APD at optimal operating state is an important condition for increasing detection sensitivity of optical receiver. Regarding this, a proper method by constant false alarm rate together with threshold detector and noise is provided. The voltages are adjusted to maintain the APD's optimal working status.