With the simulation of the flow in hydraulic machinery, experiments of cavitation, silt abrasion and their synergism on hydrofoil in the converging-diverging testing section were performed, and microscopic appearance along the damage surface was observed and depth distribution of damage was measured. The cavitation appearance shows like normal approximate round, and distributes randomly. According to the difference of action parameter on surface, silt abrasion appearance registers as wedging，relieving，ploughing, and cutting, etc. On the high particle content, the appearance of synergism mostly appears as particle wear in extreme angles (90° or 0°), and scale character on hydrofoil surface becomes more and more large than silt abrasion appearance along the position, and the depth of macro-damage is obviously bigger than single wear. On the low particle content, the microstructure probably appears as the coexistence of cavitation and silt abrasion.