Abstract: For the Passive Residual Heat Removal System,basing on the self-equivalence startup process,and adding pump controlling system,the startup method for density lock based on PID control system is advanced,that is,after the valve in PRHRS opening,the pump speed is controlled basing on the temperature changing in density lock,and the cold-hot liquid interface is maintained in the density lock,which ensures the PRHRS depart from the primary loop.According to the one-dimensional equations,the controlling startup process for the density lock is numerically simulated,and it is shown that under the effect of controlling system,the pressure in two loops can become balance gradually to start up the density lock successfully by changing the primary loop flux.The method makes the restrictions less and successful ration bigger for density lock startup.