Abstract: Relevant regulations in China and abroad specify the detailed procedures of pressure test in tightness and strength of gas pipelines, but no information about drainage of water accumulated in the pipelines after pressure test. There would be more problems in construction and operation when long-distance gas pipeline passes through complex terrain than gentle areas, including drainage after pressure test for strength and tightness. This paper discusses the principle of segmental pressure test, and presents the motion model of pig and status of slack flow in pipe during drainage. Moreover, simulation software has been developed for pressure test and drainage under different terrains on the basis of theoretical research and tests. Analog calculation is made for the field pressure test and drainage, deriving the results that fit well with field results. This software can provide a guidance to the pressure test and drainage of other pipelines.