Abstract: In order to study the entrainment phenomenon of the fourth stage automatic depressurization system (ADS-4) under real conditions, the full scale test facility for liquid entrainment (FATE) was designed and constructed in accordance with CAP1400 with the ratio of 1∶1. Boric acid solution was used as the working fluid to simulate the actual flow conditions in the reactor core. The entrainment process was recorded by a data acquisition device and a high-speed camera, and the acquired data were compared with those of the deionized water condition and the existing models. The results show that the experimental results of the boric acid and deionized water conditions are different from previous models for the entrainment onset and steady entrainment rate. The condition of boric acid is similar to that of deionized water, and the Froude number decreases with the increase of entrainment initial level. Entrainment onset is more likely to occur from top to down condition, and both have a hysteresis effect. There are also differences between the boric acid condition and the deionized water condition, which is mainly due to the different physical characteristics, especially the influence of density and viscosity.