Abstract: Test on air-water two-phase flow in transient state has been carried out in a horizontal pipe 50. 8 mm in diameter and 370 m in length. By changing air and water velocity, make it change from one balance state to another. It shows a flow pattern observed in transient state period. Comparing the data get from test with a transient state method in present we can evaluate the correction of the method. Make a simplified solution calculating two-phase flow transient state property according to different flow pattern and then forecast transient property of air-water distribution in pipe. The equation assumes an air's sub -steady state flow and partial dynamic balance. The method is applicable to the pipeline system constituted with horizontal, up dip or down dip pipe sections.