Abstract: A new passive residual heat removal system (PRHRS) based on density lock was designed. By changing mass flux in pressure-adjustment loop and maintaining high temperature water flowing in circular loop, hydraulic balance can be built in density lock, which can separate main loop away from heat removal loop. Based on AP1000 reactor coolant system, the characteristics of PRHRS on normal conditions were analyzed by using RELAP5/MOD3.2 code. It is shown that hydraulic balance can be built in density lock and the PRHRS can be started up successfully by controlling pump speed in pressure adjustment loop, and during normal conditions, if the parameters of reactor changes, the density lock can also be “locked” under the feedback control of the controlling system, which can ensure primary loop away from PRHRS.