Abstract: Weigang-Jingmen oil pipeline, with pour depressant added and transportation at lower temperature, arose wax deposit, which reduced the equivalent diameter. By analyzing the operating parameters obtained from Weigang-Jingmen oil pipeline during 1992~1998, the reduction law of pipe diameter has been found out. The paper analyzes the pros and cons of the reduction of pipe diameter, and points out that the proper reverse pumping can in effect elongate the pigging period.