Abstract: The previous studies have made some research achievements on the seepage field of a single tunnel, but the researches on twin parallel tunnel are seldom. Based on the Darcy’s law and the laws of conservation of mass, the problem of steady seepage field of twin parallel tunnels is solved using the conformal mapping technique and the method of superposition. The analytical solutions of hydraulic head distribution and water inflow of tunnels are obtained. The software COMSOL is used to verify the analytical solutions, and a good agreement is found between the two methods. Incorporating examples, discussion is given about the effects of tunnel distance on the seepage field of twin parallel tunnels, the hydraulic head distribution outside the linings and the tunnel water inflow. It is found that when the twin tunnels are simplified as a single tunnel, the hydraulic head around tunnels and the tunnel inflows would be overestimated. And the phenomenon is more obvious in case of small tunnel distances.