Abstract: For a three-lane road tunnel in surrounding rock of grade Ⅲ, the concrete invert with the thickness of 45 cm should be installed according to the requirements specified in the Code for Design of Road Tunnels. However,as per the Guidelines for Design of Highway Tunnels the invert can be eliminated. Based on Shengjie expressway tunnel project, flat and inverted arch structures in grade Ⅲ1 surrounding rock are simulated and compared using finite-difference software. The results show that the differences between the vertical displacement, vertical stress and volume of the plastic zone of the two support structure patterns are within the scope of 5%; a flat arch structure can be used instead of an inverted arch for the three-lane tunnel in grade Ⅲ 1 surrounding rock, and the flat arch structure can meet the requirements of structure stability.