Abstract: In the commissioning of a linear induction accelerator, cavity voltage waveforms measured by capacitor voltage probe (CVP) and resistor divider were almost the same when there was no beam load. The waveforms were different when there was a beam load. This phenomenon was then researched. Cavity voltage waveforms were simulated when the beam was advanced, synchronized and delayed. Simulation results showed that time difference caused by cavity structure was the main reason for the waveform differences. The distribution parameter circuit model of the accelerating cavity was created. It was shown that the beam load arrived at the two probes by different moments, which would lead to the difference of the cavity waveforms. The waveform measured by CVP was closer to the real accelerating waveform because that the CVP was nearer to the accelerating gap. The measured beam load waveforms verified that the beam load arriving times of two probes were different. The measured energy spectrum of accelerated beam was also consistent with the addition of CVP waveforms. As a result, the simulation and analysis methods in this research were effective. They would be used in commissioning and performance optimization of the accelerator.