Abstract: With passive residual heat removal system which applies the density lock as background, combining the experimental study and theoretical analysis, researches on the operating characteristics and feasibility of self-balanced startup of density lock were made in this system. The results show that self-balanced startup can be divided into two stages: Warming up stage in which the valve is closed; self-balanced stage in which the valve is open. The two requisite conditions ensuring the closure of density lock can be realized respectively in these two stages, which ensure the separation of the passive residual heat removal system from the primary circuit system by the density lock. During the stage of warming up, with the help of special structure of the density lock, the position of the transition points of the heat transfer modes is controlled effectively, and the formation of interface between the cold and hot fluids is promoted. During the self-balanced stage, with the help of the characteristics of self-stability of the hydraulic balance, the motion of interface is controlled effectively, and the hydraulic balance is established automatically in the density lock. All of the results fully prove the feasibility of self-balanced startup.