A calculation method of instantaneous geometric flow rate of an involute external meshing aviation fuel helical gear pump was analyzed in order to clarify the influence of helical gear structure on the instantaneous geometric flow rate characteristics of the pump. The sealing discharge chamber of the pump was regarded as a stack of axial multi-layer slices. The axial projection of the end faces of each layer was a single connected domain composed of piecewise smooth curves based on the method of parameterized modeling of tooth profile. The mathematical model of the boundary segment curve of each layer projection surface geometric domain was established. The area of each layer geometric domain was calculated by Green’s formula, and the volume of each layer slice was obtained. Then the volume of each layer slice was added up along the axial direction to get the total volume of the pump. The instantaneous geometric flow characteristics of the pump can be obtained by updating the gear rotation angle. Case calculations show that the average output flow rate of the pump is smaller and the flow pulsation rate is slightly increased with fixed tooth width and larger helical angle when the suction chamber and discharge chamber are reliably isolated with the unloading groove structure and with the gear backlash meshing compared with spur gear pumps with the same involute parameters on the end face. The average output flow rate of the pump increases and the flow pulsation rate increases slightly when the helical angle is unchanged and the tooth width is larger. The flow pulsation can be reduced by increasing the helical angle and tooth width when there is no unloading groove on the oil discharge side.