The failure conditions of tunnel surrounding rock under different cross section after full face excavation were studied through a number of model tests. The simulation of unsupported tunnel failure process under gravitational stress is carried out successfully in the tests. The test results show that failure of surrounding rock initiates from the upper part of the tunnel and develops upward progressively. Failure form of tunnel is different under different cross section type. Comparing with two-lane tunnel, the surrounding rock of three-lane tunnel keeps shorter stabilization, failure develops to the ground surface more rapidly, the stress changing amplitude of those beside tunnel is larger, and the rock mass on the top of the tunnel slides downward more clearly.