Tri-layer Si/mullite/Yb₂SiO₅ environmental barrier coating (EBC) was prepared on the SiCf/SiC ceramic matrix composite (CMC) by plasma spray-physical vapor deposition (PS-PVD). The EBC samples were carried out with water vapor corrosion at 1300 °C for 200 h. After steam corrosion, Yb₂SiO₅ layer forms a gradient porous structure. This is mainly due to the inclusion of SiO₂-rich layer which is precipitated from the gasification inside the coating and existing a small amount of Yb₂O₃ separately. During the corrosion process, water vapor infiltrates into the coating and reacts with the SiO₂ and Yb₂O₃ to generate volatile substances. This forms a porous structure to make the coating brittle, resulting in mud cracks finally. In addition, the results show that the Yb₂SiO₅ can react with the water vapor at the coating surface, forming an Yb₂Si₂O₇ top layer.