Abstract: For the problems of traditional rainwater pumping station，the paper introduced large diameter vacuum drainage technique in rainwater pumping station，unique advantages in application and the specific implementation methods． To study flow distribution and hydrodynamics of the drainage process，the numerical model of typical pipeline was established，using κ － ξ turbulence model and Vof method． Unsteady numerical simulation and the gas-liquid flow characteristics of the two-phase flow were analysised during water filling process in large diameter pipes． The results showed that: In large diameter drainage process，there was stratified flow，bubble flow and wave flow，and the temporary gas gathering phenomenon occured in the near elbow; liquid had a transient flow stagnation in the inclined pipe，and with the development of pressure，the liquid full pipe flow was eventually formed in the pipe．