The variable mass flow in perforated horizontal wells is very complex. One reason is that the perforation can increase the roughness of the pipe wall which will increase the frictional pressure drop. The other is the fluid boundary layer and velocity profile of axial flow will be changed due to the “mixing” of the inflow with the axial flow. The influences of the perforation parameters and flux rate on the pressure drawdown in horizontal wellbore are investigated. The perforation parameters include perforation phasing, perforation diameter, and perforation density. According to the experiment results, some modes such as friction factor calculation model (the accuracy of the model is 4%), “mixing” pressure drop calculation model (the accuracy of the model is 3%), and total pressure drop calculation model (the accuracy of the model is 2%) are developed.