In this study, the pressure drop obtained from physical experiments and theoretical approaches of a single horizontal wellbore is reviewed and a comprehensive wellbore pressure-drop model is derived for a multibranch well. We propose a new coupling model for fluid flow in multibranch wells and reservoirs. Based on this coupling model, we introduce a theoretical approach for the calculation of the pressure drop in a multibranch horizontal well with variable mass transfer. To facilitate the understanding of the physical model, the entire coupling model was divided into three parts: (1) the pressure-drop model of the wellbore, (2) the reservoir inflow model, and (3) the coupling model. By incorporating the acceleration, friction, mixing, confluence, and gravity pressure drops, a coupling model with a finite-conductivity multibranch horizontal well was developed. Newton-Raphson iterations and Visual Basic programming were employed to solve the coupling model and to obtain the pressure and the inflow rate of the wellbore. The wellbore pressure-drop model was verified by comparing it with different models for the same case study, which has been previously introduced in a different research work. Furthermore, the forecast and sensitivity analysis were conducted, and then the results are discussed. In the proposed new model, several factors are considered, including the wellbore structure, the wellbore completion method, the wellbore, and the fluids and formation properties. The presented approach can be used as a valuable tool to analyze the influence of the pressure drop on the productivity of complex-structured wells and vice versa, and to quantitatively investigate the various pressure drops in wellbores, including the friction, acceleration, mixing, confluence, and gravity pressure losses.