The performance of deflector jet servo-valve is extensively influenced by the flow field characteristics inside the pilot stage of the deflector jet servo-valve. Pressure oscillations and cavitation phenomena commonly arise in the deflector jet pilot stage that usually lead to high frequency noises and vibrations of the servo-valve. To investigate this issue and understand the internal flow characteristics of the pilot stage, a three-dimensional numerical simulation is carried out in this paper. The simulations are carried out using the commercial ANSYS/FLUENT 18.2 CFD software. Large Eddy Simulation (LES) coupling with cavitation model is utilized to simulate the pressure oscillation phenomena and capture the unsteady cavitation dynamics under several inlet pressures. The result shows that the pressure oscillation phenomena intensified by the increment of the inlet pressure. Moreover, the characteristics of generating, growing, and merging cavity shedding phenomena have been identified inside the pilot stage of the deflector jet servo-valve. Finally, it can conclude that pressure oscillation may one of the root cause of cavitation in the deflector jet pilot stage.