Deepsea hydraulic oil power source is the most common power source in deepsea at present. It was introduced of the hydraulic oil viscosity properties and its influence on the hydraulic system by started from oil used in the deepsea hydraulic oil power source. The change of the hydraulic oil viscosity was analyzed emphatically under high pressure and low temperature environment in the deepsea, and the computer flow dynamics (CFD) software was used to simulate with analysis internal flow field of deepsea power sources gear pump. It is obtained of the internal viscosity distribution of gear pump and the viscosity influence on the pump efficiency, which provides scientific gist for the option of power sources hydraulic oil in the deepsea.