Abstract: On the basis of experiment and research on the tow-phase horizontal flow of Gas-Power Law Liquid, classification is done according to two phase medium, the flow state of gas and liquid two phase flow is classified as; bubble flow, stratified flow, wavy flow, slug flow, impacted flow and annular flow.The flow state is the main factor for defining pressure drop rule of two phase flow.The writers went further into the problem of the mechanism of different flow states giving the method of calculating the pressure drop of differeat flow states. There is obvious interface between gas and liquid for stratified and wavy flows.Flow separating models can be adopted.As for bubble flow, monophasic flow models can be used for there is no sliding between the two phases.Reguarding slug flow and impact flow, definition but dimensionless pressure gradient method can be chosen because the flow mechanism is rather complicated.And for annular flow, pressure drop value can be available by correlating the simultaneous coefficient of split gas phase and Lockhart and Martineli parameters. The said method provides a reliable hydraulic calculation not only for the design of unheated oil and gas transportation but also for petroleum and chemistry industry.The average absolute error of this method, compared with other methods is very small and high in accuracy.