Abstract:The property changes of the working medium of fire resistant oil will affect the operating property of the electro-hydraulic servo valve. According to the phenomenon of the valve core jammed after replacing the fire resistant oil in a power station, the foreign matter analysis, mass loss analysis, mix oil test analysis and so on multi source searching method were used to analyze the mechanism and effect causes of the servo valve jammed failure. Comprehensive analysis of the test results show that the servo valve jammed failure is mainly due to the oil sludge generated from the oil change of the hydraulic system, which blocks the slight gap between the valve corn and the valve body valve, increases friction and causes jam. The main source of sludge is metal particles, the degradation components of fire-resistant oil, the dissolved products of rubber pipes and foreign pollution products such as diatomaceous earth and adsorbent and so on.