The gas phase accelerating beyond the liquid phase caused by gas-liquid slippage cannot be ignored in short horizontal pipelines with undulation and inflow, and there is no method to calculate it. Therefore, a pressure drop prediction model for variable liquid holdup was developed in this paper. The theoretical model calculation results were validated using computational fluid dynamics. The effectiveness of the pressure drop prediction model has been demonstrated. The various pressure drop, liquid holdup, and development length laws were then examined. The findings indicate that: the pressure drop in the developed section of stratified flow is not only the friction pressure drop but also the acceleration pressure drop; the length of the stratified flow development section and pipeline pressure drop are more easily affected by the flowrate than the liquid holdup in the pipe inlet. Using the relevant data from coalbed methane horizontal wells as an example, the L/D of the development section is approximately 40–85 when the inlet flowrate is 0.8–1 m/s, and the inlet liquid holdup is 0.3–0.5. The pressure drop characteristics in the gas-liquid stratified flow development section are obviously different from those in the stable section. The development of a pressure drop prediction model for the stratified flow development section lays the theoretical groundwork for the investigation of gas-liquid two-phase flow in horizontal pipelines with short or undulating and inflow conditions.