Aiming at non standard octagonal gasket used in hydrotreating reactor. Ansys was used to calculate the contact stress on the sealing surface during the hydrostatic pressure test, and the causes of the different contact stress on each sealing surface were analyzed. The results showed that the contact stress down to the trough when the test pressure was the highest, the maximum stress on the inner side of gasket was away from the center of gasket, and the outer situation was in contrast to it. Variation of gasket sealing surface contact stress provides a reference to the analysis and evaluation of other non standard seal structure.