It is a long-term goal and of great significance to seek a new method towards direct realization of the SI unit of sound pressure and no longer rely on the sensitivity of standard laboratory microphone in the field of airborne acoustic metrology. Laser Doppler velocimetry technique can be used to realize the goal. In this paper, the optical interferometer without Doppler frequency bias is designed to investigate the realization of sound pressure in traveling wave tube with the photon correlation spectroscopy (PCS). According to the linear relationship, the sound pressure of plane acoustic wave can be acquired by the demodulated particle vibration velocity from the Doppler signal. The experiments are also implemented to demonstrate the feasibility and accuracy of this scheme with the working standard microphone as reference. For the acoustic frequency of 315 Hz, the deviation between the two methods is less than 0.5 dB with the measured sound pressure level varied every 1 dB in the range from 100 dB to 110 dB. For the measured sound pressure level of 105 dB, the deviation is less than 0.3 dB with acoustic frequency of 315, 400, 500, and 800 Hz.