Herein, the optical path difference (OPD) distribution of supersonic semi-free jet under different total pressure conditions was investigated in a vacuum chamber to address the problem of reduction of image quality attributed to the non-uniform distribution of jet refractive index field. The angle between the main optical axis of the three optical paths and the flow direction was 81°, 90°, and 99°. Experimental results show that larger total pressure corresponds to larger OPD of the jet. Under the same total pressures, the OPD of the 81° optical path is larger than the 99° and 90° optical paths. An empirical polynomial of the OPD along the flow direction was deduced, and the applicability of two theoretical models in the jet flow field was also analyzed.