Wheeled in-pipe robots have been widely used in pipelines inspection and exploring because of simple structure, steady performance and consuming less energy. However when wheeled mobile robots run in muddy in-pipe environment they often can't be control to move accurately and continuously mainly because the ground cannot provide adequate adhesion. In order to improve the movement performance of the robot in this environment we establish a model based on terramechanics. With this model further research including motion performance of wheeled in-pipe robots has been conducted in muddy in-pipe environment in this paper. The model of wheel-terrain interaction is established with the finite element software ABAQUS. With this model we analyze what and how the factors which affect the adhesion. The actual experimental results show the model and simulated analysis is useful and helpful to mechanical design and control strategy design for wheeled in-pipe robots.