Abstract: Based on the passive residual heat removal system (PRHRS) with density lock, the validity of self-equivalence startup for density lock was proved by experimental study. It is shown that the cold-hot liquid interface will move up or down because of the unbalance pressure between the PRHRS and primary loop during the startup process when the primary mass flow is close to the balance flow, which will make the gravity pressure difference in PRHRS become lower or higher, and the cold-hot liquid will be balance in new position. Finally the density lock starts up by self-equivalence, and PRHRS is separated from the primary loop. The Matlab code was used to analyze the startup characteristic of the density lock, and the process of self-equivalence startup for density lock was proved reasonable and effective. The numerical calculation results agree well with the experimental data, and the code is well to analyze the transient phenomenon during the self-equivalence startup process for density lock.