Abstract: This paper shows causes of constant impurities occurred in pipeline by analyzing rust and origins in pig impurities of products pipeline, that is, the internal corrosion of pipeline was brought by water deposition in lower location of pipeline during its hydrotest in pipeline commissioning period. It brings out dewatering methods for oil carrying water. To study the flow pattern of oil carrying water system, distribution and volume of water deposition, experimental study on the movement form of water deposition flushed by oil flow and the water volume greater than zero at critical condition are made with transparent tube and steel tube. Diesel oil and water are used in order to observe the distribution shape of water deposition by means of a 15 mm (inner diameter) glass tube comprised of downward, level and upward testing sections and a 25 mm (inner diameter) plastic tube experimental system. Meanwhile, a 27 mm (inner diameter) steel tube is used as the experimental tool to measure the water volume in different position of upward pipe. Water deposition tends to move forward by oil carrying in a form of wall-closed exocentric big water drop when velocity of oil phase is comparatively slower; increasing velocity of oil phase will scatter the lower part of exocentric big water drops into small ones; the bigger the flow rate of oil is, the longer the distance of big water drop travelled is and the higher the water volume is.