Abstract: The fourth stage automatic depressurization system (ADS-4) is an important passive safety device in AP1000 which enables controlled depressurization of reactor coolant system in small break LOCA. However, the large amount of coolant can be carried to the containment via ADS-4 entrainment and the upper plenum entrainment during the depressurization process, yielding reactor core uncovered and melted. In order to investigate the ADS-4 entrainment depressurization and the upper plenum entrainment processes in small break LOCA, the entrainment and depressurization features were investigated in the experimental loop with scaled ratio 1∶5.6 to AP1000 under different initial pressures, mixture liquid levels in the pressure vessel and heating powers. The entrainment deposition effect of reactor internals was also investigated. The experimental data reveal that the large amount of water is entrained through the ADS-4 branch line within a short period of time. The liquid entrainment rate and the decrease rate of the mixture liquid level in the pressure vessel increase with the initial system pressure. It is notable that the core uncovery is experienced when the initial pressure is set as 0.5 MPa in current experimental conditions, but the reactor internals have little effect on the entrained mass and the mixture liquid levels in the pressure vessel.