For the large span and flat, together with the interaction effect of construction and disturbance of surrounding rock, the mechanical of large span and flat multiarch tunnel is very complicated, and the timespace effect is different from that of twolane multiarch tunnel or separated tunnel. Adopting the “CTSSSRH”, the 3D dynamic process of the tunnel construction was simulated. Analyzing the rule of surrounding rock displacement and stress with excavation, the timespace effect of large span and flat multiarch tunnel was proposed.