In order to analyze the characteristic of the unsteady pressure in the internal gear oil pump, based on the standard k-ε turbulence model and the immersed solid model, the three dimensional (3-D) unsteady calculation was carried out about the internal flow field of the gear oil pump by the software CFX of flow field analysis. The monitoring points were set up in the different locations of the internal gear pump, and the results about the pressure pulsation were obtained, and then those data were analyzed in frequency domain. Results show that the meshing segment in gear cavity is the main vibration source, the pressure signal decays gradually passing from the meshing segment to both sides of the inlet and outlet. The pressure amplitude in each gear cavity grows gradually along circumference direction from inlet to outlet. The pressure pulsation amplitude of the monitor in driver gears cavity is much higher than the value in driven gear. The dominant frequency of pressure pulsation is 175 Hz in inlet/outlet monitoring points, and 350 Hz in the diver gears monitor, which is the main factor of influence to the pressure pulsation of gear oil pump.