Abstract: The full scale test facility for liquid entrainment (FATE) was constructed in accordance with large-scale advanced pressurized water reactor with a 1∶1 scale ratio in order to investigate the entrainment phenomenon of the fourth stage automatic depressurization system (ADS-4). Multiple groups of entrainment tests were carried out under different working conditions. The experimental data were compared with the results of different models, and the experimental data were fitted to obtain the entrainment onset model. The results illustrate that the phenomena of the quasi-real and deionized water working conditions are similar, and the Froude number of the branch pipe increases with the gas chamber height of entrainment onset. The hysteresis phenomenon appears in the combination of different working conditions. Due to the effect of viscosity and surface tension, the entrainment onset in the quasi-real working condition is more difficult than that of the deionized water condition. In the case of deionized water, boric acid or impurity condition, most of the test data of steady entrainment rate are quite different from those of Welter’s model. Except for a few test data, the relative errors are greater than 20%.