The optical method of photon correlation for the measurement of the free-field acoustic pressure has meaningful applications, such as forming a traceability chain based on the absolute measurement for various types of acoustic sensors. The optical method based on photon correlation has been considered to be a potentially new primary standard that overcomes the limitations of existing acoustic standards based on microphones. The key issue is accurately obtaining the spacing of the interference fringes or equivalently the crossing angle between two laser beams. Usually, the trigonometry method is used to calculate the crossing angle, which imposes limitations on the accuracy of the measurement. Optical microscopic imaging is introduced to directly measure the spacing of the interference fringes in the acoustic pressure measurement based on photon correlation method for the first time. We propose directly measuring the spacing of the fringes where the standard resolution target is placed in the interference region to produce the superimposed image with the fringes and recording the results by the CCD. Both the calibration of magnification and the measurement of the fringe spacing are performed through the same single-shot image. The uniformity of the interference fringes is analyzed by numerical simulation, and the optical configuration is optimized according to the influence of the crossing angle of the two Gaussian beams. The experimental system is built based on the photon correlation method. The experimental results show that this method improves the accuracy of the acoustic pressure measurement by 0.03 to 0.82 dB in the frequency range of 0.5 to 4 kHz in free-field conditions.