The problems like cracking of the girder in the mid-span and the ever-increasing vertical deflection appear during the long term usage of the long-span continuous rigid-frame bridge． Post-tension tendon with reserved duct can increase the pre-stress of the main beam effectively，and decrease the long term span deflection in order to improve the performance of the girder． At the same time，the proper tension position is very crucial to optimise the stress distribution of the bridge and control the deflection increase． Combining with practical engineering， the authors analyze the influence of different positions of post-tension tendon ( including top-，weband bottom plate tendons) on the stress and deflection of the main beam，and find out the optimal position of post tendon．