Aiming at the problem that the theoretical derivation of the change of sound pressure at the receiving end in the sound velocity measurement experiment is not perfect, especially the unclear explanation of the principle for the phase comparison method, a multiple reflection model based on sound pressure equation is established from the relationship between the amplitude and the sound pressure. In the model the plane simple harmonic wave function, the formula of sound pressure reflection coefficient and the principle of wave superposition are used to derive the formula of the change of the amplitude and phase of sound pressure at the receiver with distance, and the principle of the standing wave method and the phase comparison method are explained. The simulation results show that the amplitude and phase of sound pressure at the receiver are not strictly periodic functions, and the measurement accuracy is related to factors such as the sound absorption coefficient and the sound pressure reflection coefficient. Finally, the experimental verification is carried out, and the experimental measurement results are in good agreement with the calculation results of this model. This study provides theoretical guidance for correctly understanding the physical nature of the experiment.