Electrostatic discharge was encountered in the pressure oil system of governor under the unloaded operation condition for a large hydropower plant, therefore, it is important to better understand the oil flow state inside the double- cartridge filter in the system to reveal the mechanism of electrostatic charge generation. A porous medium model was used to describe the fluid dynamic characteristics of the filter element, and the viscous resistance and inertial resistance parameters in porous zone were obtained by experiment in actual operating condition. And then a numerical model was established to simulate the oil flow inside the double-cartridge filter. The flow characteristics inside the filter was analyzed and the influence of the inlet velocity is discussed in detail. The correctness of the numerical model was demonstrated by experiments. The results show that the pressure loss inside the filter increases with the increasing inlet velocity. The oil velocity is relatively slow, and the change is insignificant within filter cartridge. There are symmetrical recirculation and stagnation zones at the bottom of the filter. Due to obstruction of filter element, the oil pressure gradually decreases from the inner wall to the center of the cartridge. The pressure loss caused by the filter element increases with the increase of the inlet velocity.