The calculation model on water hammer and pipe vibration are built to discuss the water hammer and pipe under the ultra-supercritical steam trap during the valve-opening process. The research is based on fluid-structure interaction theory of liquid-filled pipe. The time domain curves of water hammer pressure on the fluid, fluid velocity, pipeline axial internal force and pipeline axial vibration velocity are obtained under different flow characteristics and sleeve layers. The analysis shows that: water hammer pressure is mainly depended on the interaction of flow rate and pressure; the pipeline axial internal force, locally influenced by pipeline axial vibration velocity, is greatly influenced by water hammer pressure. At the same rated flow, the peak of the water hammer pressure under linear flow is significantly less than that of quick-opening flow, while the flow velocity under linear flow is greater than that under equal percentage flow. Thus, linear flow characteristic is best for the ultra-supercritical steam trap. The more layers the sleeve has, the lower the peak of water hammer pressure and pipe axial internal force are when the velocity fluctuations and pipeline vibration increase.