Aiming at the problem that the avalanche photo diode(APD) gain changes with temperature under the same voltage, which affects the dynamic range of the optical time domain reflectometer(OTDR), an adaptive precise control scheme of APD bias voltage is proposed. At each suitable operating temperature, This scheme can ensure that the bias voltage is always close to the avalanche voltage, and make APD work stably at the best gain, so as to improve the dynamic index of OTDR, and solve the problem of large dynamic range fluctuation in the high and low temperature test of OTDR, so that the dynamic range fluctuation is controlled within 0.3 dB.