Abstract: Based on the passive residual heat removal system（PRHRS）,the closure of the density lock on normal condition and the startup of the density lock on accidents are analyzed by experiments,and the iN-Stantaneous characteristics of PRHRS after accident are simulated by RELAP/MOD3.2 code.The calculated results are compared with the experimental data,and they agree well.It is shown that on normal condition the density lock can separate the heat removal loop from the main loop,and the PRHRS does not work during this condition,while when the accident happens the PRHRS can operate instantly by different gravities in two loops and it will set up the steady natural circulation between the heat removal loop and the main loop to re-move the residual heat gradually.