Abstract: The basic theory and experimental results of measuring diode voltage with differential absorption method were reported. With the MCNP model of 'bremsstrahlung-attenuation-detector' system, the relation curve of the radiation dose and the diode voltage was fitted. The measurement system of diode voltage with differential absorption method was established, and the waveforms of diode voltage were gained by putting absorbing films of different thicknesses in front of the detectors. By using the fitted curves and the experimental waveforms, the diode voltage of Chenguang accelerator was obtained through iterative method. The peak value of diode voltage was 0.58 MV. The results derived with differential absorption method agree with those measured with the conventional method.