Worldwide, the pipeline mileage has increased dramatically since 1948 when the industry began to keep mileage statistics for pipeline construction, especially in China. Before operating long distance oil and gas pipelines, the pressure testing for tightness and strength is of great importance for ensuring operating safety. Water is used as service fluid for the pressure testing due to its safety, and long distance pipeline is divided into a number of small sections according to complex terrain conditions. Segmental water pressure testing is now very frequently used in the oil and gas industry, ensuring safety and efficient construction of pipelines. However, some new problems may arise, in practice, for the pigging process, following water pressure testing. Based on the review of previous papers about pipes accidents, the causes can be classified into hydraulic and nonhydraulic aspects. With the method of characteristic (MOC) and basic theory of gas–liquid two-phase unsteady flow, a mathematical model is developed to simulate the hydraulic transients during the pigging process. The model has been applied to some segmental pipes in China to predict varying pressures under complex terrain conditions. Pressure pulses predicted at the end of pigging in the numerical results have also been found to occur in field trials. The analysis shows that pressure pulses may cause overpressure accidents due to vapor cavity collapse. The techniques in this paper can give reasonable instructions in long distance pipeline constructing, promoting the development of Chinese oil and gas industry.