The invention relates to a pressure vessel flange separating mechanism which comprises an upper sleeve, a lower sleeve and a transmission shaft, wherein the upper and lower sleeves are symmetrically arranged; the transmission shaft penetrates through the upper and lower sleeves; the upper and lower sleeves are fixedly connected with an upper flange and a lower flange of the pressure vessel through an upper connector and a lower connector correspondingly, and the mechanism further comprises a transmission nut which sleeves the transmission shaft, is fixedly arranged in the lower sleeve and is in threaded connection with the transmission shaft; a supporting part used for being propped against the upper sleeve is also arranged on the transmission shaft; when being rotated forward, the transmission shaft is screwed out of the transmission nut through the threaded transmission property; and through the supporting part arranged on the transmission shaft, the upper sleeve is driven to move upward, thereby separating the upper and lower flanges from each other. Compared with the prior art, the pressure vessel flange separating mechanism has the advantages that all that is needed for separating of the upper and lower flanges is to connect the whole pressure vessel flange separating mechanism with the upper and lower flanges of the pressure vessel; the height of the pressure vessel flange separating mechanism is reduced when the operability is guaranteed; and accordingly, the floor space of the whole pressure vessel flange separating mechanism is greatly reduced.