Through the stress analysis of bellow mechanical seal,the reasons of the force fluctuation on the bellows were discussed and the calculation formula of the stress was proposed.The analysis shows that,when the movable ring is rotated at high speed,the varied dynamic pressure is produced for the fluid between the movable ring and stationary ring because the movable ring and stationary ring is mot parallel due to the structure of the bellows and manufacturing and installation error.The fluctuation of the dynamic pressure is the main cause of fatigue failure of the bellows.The static stress of the J113-070/UBEGCF bellows was analyzed by ANSYS software,the node of maximum stress was identified and its life was calculated.