Abstract: A recirculating controlled water chemistry system was employed to simulate the primary circuit water environment of a pressurized water reactor (PWR). High-temperature and high-pressure water corrosion tests were conducted on SiC/SiC_f composite cladding tubes. The corrosion performance was investigated through mass change measurements, microscopic morphology analysis, and composition characterization. The results indicate that the corrosion process of SiC/SiC_f composite cladding tubes in high-temperature high-pressure water could be divided into three distinct stages: a corrosion mass gain stage, a transition stage (mass gain to mass loss), and a corrosion mass loss stage. During the corrosion mass gain stage, oxide particles and clusters continuously grew and interconnected, forming an oxide film that reduced the contact area between the substrate and the high-temperature high-pressure water, thereby slowing down the corrosion dissolution of SiC. In the transition stage (mass gain to mass loss), oxide particles or clusters gradually disappeared, exposing angular crystals and corrosion pits on the surface, indicating that the dominant mechanism shifted to SiC dissolution in high-temperature high-pressure water. In the corrosion mass loss stage, the surface of the SiC outer coating became smooth, with no visible corrosion pits, and the corrosion rate was stable.