A unitized regenerative fuel cell is a typical gas liquid two-phase system that consumes oxygen and hydrogen in a fuel cell mode and liquid water in an electrolytic cell mode. Liquid water removal is crucial for a successful cell start-up after the electrolytic cell mode ends. However, investigations on two-phase transfer mechanisms for liquid water removal are limited during mode switching. To fill this research gap, a three-dimensional two-phase full-cell model is developed to describe charges, gas mixtures and liquid water transfer corresponding to operational modes of electrolytic cell, gas purging and fuel cell. The cell is assumed to being in the isothermal state. Numerical model is analyzed by using COMSOL Multiphysics 5.3a software. Furthermore, experimental and simulated results are compared to validate the proposed model. Results show that more than 84.0% of pore volume is occupied with liquid water on porous layers in an electrolytic cell mode. Although pre-switching for water removal can decrease the volume fraction of liquid water within the porous layers from 0.88 to 0.50 in a short time, more time is required for liquid water being carried away under a low level of water flooding. Purging result presents diverse influences on start-up performance in a fuel cell mode. Liquid water distribution, which is similar to that in a fuel cell mode, formed in a purging mode is encouraged for promoting a quick and stable start-up in a fuel cell mode.