Deflector jet pressure servo valve is a typical complex electrohydraulic servo system. Oil contamination can cause the increase of oil level during service and result in progressive degradation of servo valve’〖KG-*2〗s performance and even finish endurance life. Aiming at this problem, the working principle of the deflector jet pressure valve was discussed and its mathematical model was established. The pollution stagnation mechanism was analyzed and the friction model was established.The quantity and diameter distribution of oil level in GJB420B were analyzed and the frictional force was calculated. Finally, the whole valve model was built to simulate the performance degradation of the deflector jet pressure valve under different oil level. The results show the frictional force of the pressure valve gradually increases with the increase of the oil level，which can finally cause the decay of servo valve output pressure and response slowly. The research methods and results have important reference value for qualitative analysis, prediction of pollution stagnation fault and life extension under severe working conditions.