Conceptual design of advanced passive residual heat removal system in which density lock is applied is proposed, moreover theoretical analysis and experimental study on its feasibility is carried out. According to the design, atmospheric-pressure experimental loop is built so that researches on working performance of density lock in the system are done at steady-state operation and pump trip conditions. The results show that at steady-state operation conditions, density lock can keep close in a long run, which will separate passive residual heat removal system from primary circuit system. As a result, passive residual heat removal system is in the non-operating conditions, which do not influence normal operation of reactors. At the pump trip conditions, density lock can be automatically opened quickly, which will make primary and passive heat removal system communicated. The natural circulation is well established in the two systems, and is enough to ensure removal of residual heat. In addition, experimental study on operating reliability of density lock is made on the condition of small flow surge. The result show that the flow surge do not lead to the failure of the function of density lock, but bring about a little position change of temperature interface in the density lock, moreover, a new balance steady state will arrive very soon.