Abstract: Topological optimization of gathering pipeline network is an important part of the optimization of surface gathering and transportation system in oil and gas fields, and different topological structures have direct effect on the economic benefits of construction and development of gathering systems. At present, the researches on the topological optimization of gathering and transportation pipeline network focus on the dendritic pipeline network, radial pipeline network and loop pipeline network, but less on the optimization of composite gathering and transportation pipeline network with one loop connecting with multiple branches. In this paper, a new optimization method was developed based on a method of searching the optimal loop. In this method, each wellhead is interpolated into its nearest loop network to form a composite looped-dendritic pipeline network with one loop connecting with multiple branches. Then, the effects of multiple factors(e.g. topographic relief) on the pipeline route were taken into consideration to study the topological optimization of composite looped-dendritic pipeline network at 3 D terrain with the least pipeline cost as the objective function and the related gathering conditions as the constraints. Besides, a new algorithm combining the coupling genetic algorithm with the A* algorithm was proposed to resolve the optimization model. Finally, the gathering and transportation pipeline network of a certain CBM field described in the literature was taken as an example for model verification. It is shown that the connection length of looped-dendritic gathering pipeline network is cut down and investment cost of the network construction is reduced by virtue of the pipeline network connection pattern that is optimized by using this new method. This method can not only be used to realize the optimization on two-dimensional plane, but also be used for the optimization at 3D terrain after the terrain is simulated.