Hydraulic gear pump is composed of a pair of intermeshing gear, through the gear meshing and separation of the tooth rotation to form a change in the volume of oil and oil pressure. When the gear meshing, the meshing between the two teeth of the hydraulic oil due to the closure of the teeth can not be discharged to form a trapped oil phenomenon. Trapped oil will produce high pressure, side pressure on the shaft, easy to bend the shaft, bearings premature damage, but also consume the motor power. The solution is to open a discharge groove to the discharge chamber at the side of the gear meshing so that the oil trapped between the two teeth can be discharged to eliminate the trapped oil phenomenon.