Abstract: Horizontal well production is one of the most challenging new research projects in petroleum engineering. It is very clear that horizontal well has its advantage in the development of viscosity oil, low permeable, fractural, bottom water, gas cap reservoirs and in the applications to dig out oil production potential of developed wells. Oil production rate is influenced greatly by the pressure drop along the well bore, because of longer distance of horizontal interval and higher pressure drop in the well. Therefore, it is very important to have a better understanding of mechanism for flow loss and pressure drop along the horizontal interval, and the principle can be used to optimize horizontal interval, to design production technologies and to predict production rate of horizontal wells. In a real horizontal well, the fluid flow from different perforations is added to the main flow along the horizontal interval continuously. This kind of flow is variable mass flow and its flow mechanism is more complex than that of normal tube flow. This paper describes numeric simulation approach which is used to study the flow pressure drop and influence of friction caused by the injection from well wall. It also supplies single hole flow and summarizes the relationship among the pressure drop, injection rate from perforations and well bore section Reynolds number. The loss of multi-hole flow will be introduced in Part two, Muti-hole flow of Numeric Simulation Study of Flow Loss for Variable Mass flow in Horizontal Well.