The internal flow field of direct drive cartridge valve was analysed by fluid mechanics software Fluent, then the pressure and velocity distribution nephogram of valve internal fluid in different opening degree were obtained. As the result, the causes of the energy loss of liquid flow were analysed and the structure parameters were optimized. The results show that: the total pressure gradient is smaller if the valve opening degree is smaller, and the velocity gradient of fluid in the valve is nonuniform, only the throttle position changes greatly; the total pressure gradient maximizes when the opening degree reaches the maximum, and fluid maximum speed increases; when the opening is 20 mm, the fluid velocity gradient in the position with smaller pressure gradient is also smaller, and the fluid velocity gradient in the position with larger pressure gradient is larger too; The energy loss mainly because a vortex regionthe with the diameter about 10 mm appeared in the oil port aperture of valve sleeve.