To study influence of run-time on dynamic characteristics of water/oil separation, a mathematical model is established to describe flow field in vacuum oil purifier. Evaporation of droplet phase transition equations are established, considering effect of three-phase flow of oil/water/vapor and water droplets evaporation phase change. Influences of run-time on oil/water/vapor three-phase volume fraction distribution and axial dehydration rate are analyzed. Dynamic characteristics of oil/water/vapor three-phase flow at different run-time is indicated. Efficiency of dehydration of water/oil separation is enhanced with run-time. It provides a foundation for further research on water/oil separation mechanism in vacuum oil purifier.