The invention discloses a process for repairing water erosion damage of a flow passage component of a steam turbine and protecting the flow passage component of the steam turbine. In the process, molded lines of the flow passage component of the steam turbine are repaired by adopting copy overlaying of manual argon tungsten arc welding aiming at the phenomenon of the water erosion damage existing in the flow passage component of the steam turbine. A water erosion resistant coating is prepared by supersonic speed flame spraying so as to achieve the purposes of repairing the molded lines of the flow passage component of the steam turbine and protecting the molded lines at large area on line and prolonging the service life of the flow passage component of the steam turbine. Through the process of the invention, the flow passage component of the steam turbine can be repaired and protected at large area on line in a short time without changing vibration characteristic, steam-driven characteristic and strength characteristic of the flow passage component of the steam turbine, so that the process has the advantages of high on-site operability, small deformation of the repaired workpiece, remarkable improvement on water erosion resistance and the like, can solve the common problem of water erosion in the flow passage component of the power station steam turbine in service on site, improves the running safety and efficiency of units and increases economic benefit of a power plant.