Abstract: A deformation mechanism method is proposed based on the concept “tunnel-slope system”, taking the outlet section of the shallow tunnel under unsymmetrical pressure of Hanjiamo tunnel No. 3 as the study object. The engineering geological conditions are investigated by using this method. The structural type and deformation mechanism of the system are studied considering the deformation characteristics and monitoring achievements during the construction process of the outlet section. FLAC^3D is used to simulate the excavation and support processes of the outlet of Hanjiamo tunnel No. 3 under the condition of shallow cover and unsymmetrical pressure. Some conclusions are drawn through analyzing the characters of stress and displacement fields and plastic zone of surrounding rock after excavation. It will be useful for the project construction.