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The deflector jet servo valve, whose static and dynamic performance can be deteriorated by the generated flow cavitation phenomenon, is a vital segment in achieving precise control of electro-hydraulic servo valves. Thus, the distribution of the flow field in the pilot stage of the deflector jet servo valve has a serious impact on the performance of the valve and even the capability of the entire system. This paper is mainly presented in the three-dimensional modeling and simulation of the pilot stage flow field. For observing the characteristics of cavitation, Eulerian multiphase model, standard k-ε turbulent model, and vapor fraction transport equation model are used. The result shows that the increment of inlet pressure intensifies cavitation in the area of null clearance of the deflector jet amplifier segment and at the edge of receiving ports A & B. Moreover, large eddy simulation is used to calculate turbulent characteristics and the governing equations are solved for transient state condition to capture the characteristic of generating, growing, and merging cavity shedding phenomenon. The numerical cavitation distributions are simulated inside the deflector jet pilot stage at different oil viscosities. The result concludes that cavitation phenomenon is intensified with the decreasing of fluid viscosity. It is also observed from the numerical analysis that output pressure plays a significant role in controlling cavitation intensity around the pilot stage of the deflector jet servo valve [ABSTRACT FROM AUTHOR]