A more practical method to demonstrate the principle of sound speed measurement experiment is proposed. Sound pressure of received side is deduced from the method of small surface element integration. Sound pressure obtained from spherical wave and plane wave source are calculated for comparative study. Compared with the size of the transducer，the distance between the transmitting end and receiving end can not be regarded as infinitely great or infinitely small. In this specific measurement range，the tendency of pressure change with respect to distance obtained from the proposed method is closer to the data measured from experiment. The difference between calculated and measured data is also reduced to less than 1 /10 compared to the last two modes.