Aiming at the deficiencies of tubing-conveyed oriented perforating technology for horizontal well, a pumping perforation technology in horizontal well has been developed. Pumping perforation technology process is a perforation process transferring the perforating gun string into perforation section of the horizontal well with pumping fluid, and then completing the perforation while perforating bullets exploding after powering on, and finally towing back the perforating gun to the ground with cable. It is of the advantages such as simple perforation column, simple and convenient process and the shorter operating time. Perforating gun string is acted on by the impact, gravity and buoyancy of pumping liquid during moving in the horizontal section. Based on the theory of metal friction and the hydraulics principles, the calculation model of the critical injection rate (namely pumping displacement ) in the horizontal section is established by considering the effect of the composition of the gun string, buoyancy, friction coefficient, heavy guns, gun ramps, pumping fluid density, and calculated the impact of impact, effects of assembly components of gun string, pumping-fluid buoyancy, frictional coefficient, weight of gun, angle of gun-string, pumping-fluid density and computing method of impacting force. Field applications show that the calculation model established can meet the construction needs.