The mechanical seal of the underwater vehicle’s stern shaft was taken as the research object,the finite element method was used to analyze and contrast the sealing performance of the mechanical seals with different seal surface width under different diving depth.The influence of the mechanical seal surface width on the sealing performance under the special conditions such as great diving depth and low speed was invesgiated.The results show that when the sealing surface width is constant,the maximum temperature,contact pressure and axial stress of the sealing surface are increased linearly with the increasing of diving depth.When the diving depth is certain,the greater the width of the sealing surface,the higher the maximum temperature of the static and dynamic rings,the faster the maximum contact pressure and axial stress will increase.Under the same working conditions,the leakage is reduced with the decreasing of the width of the sealing surface.This research shows that under the special conditions of great diving depth and low speed,the selection of smaller seal width is beneficial to reducing the maximum temperature,reducing the maximum axial stress and contact pressure,and reducing the leakage.