Abstract: The transition is a section of riser system of offshore platform. Because of the expansion caused by pipeline temperature difference and internal pressure, there exists displacement in riser's transition termination, which will affect the total riser system. If the displacement of riser's transition termination is too large, the transition will contact with platform and damage to riser system and platform will take place. As a result, the effect of transition on riser system must be considered while analyzing riser, and the length of transition must be determined in the first place while considering the effect of transition. Considering double-layer structure adopted commonly in marine pipeline at present, the transition length of double-layer pipeline is calculated by integral equivalent method in this paper, and calculating equation of transition length is deduced according to the above method. Through a computational example, it shows that the computational equation of transition length of double-layer pipeline is reliable, and can provide theoretical dependence for engineering design in calculating transition length of buried double-layer pipeline.