Aiming at the high-frequency oscillating phenomenon appeared in the use of threestage electro hydraulic servo valve,through theory analysis and practical experience，the failure cause of the highfrequency oscillating phenomenon of the threestage electro hydraulic servo valve was revealed. Because friction resistance between the valve core and valve sleeve of the threestage electro hydraulic servo valve became larger, the pilot valve static hysteresis was enlarged, so the response speed and control accuracy decreased, influencing the working stability of the servo hydraulic system.According to the test curve,corresponding repairing was made to ensure equipment stable operation.