In order to calibrate the drift of PMT used measure, the intensity of the reflected or transmitted light, the incident beam was divided into two beams, measuring beam and reference beam. Passing through the same mechanical chopper, the reference beam was chopped into 3. 2KHZ light pulse beam while the measuring beam into 200HZ light pulse beam. By the same PMT, both the light pulse beams were transformed into electric pulse signals, which were then amplified by the same preamplifier and VCGN. The reference pulse signal and the measuring pulse signal separated from each other by the bandpass fillters were rectified to DC signals, and a constant was subtracted from the reference DC signal by subtracter. After the remainder signal sent out from the subtracter was further amplified, it was used as controlling voltage of VCGN. The drift except the filters, the subtracter and the DC amplifier could be automatically calibrated by VCGN in real-time. The output variation of the calibrated reference signal was less than 0. 01% when the reference signal that came out of PMT varied from 1mV to 980mV. The output instability of the measuring signal was less than 0. 1% in eight hours. A highly stable light source supplied the reference beam for calibration PMT drift used to measure the intensity of the emitting light.