The mechanics behavior and surrounding rock failure law of small-spacing tunnel under construction with unsymmetrical loading were studied via finite element method. The construction process (with CD (Center Diaphragm) construction method) with varied excavation sequence and tunnel distance was dynamically simulated and analyzed in ABAQUS. The rule of the plastic damage of surrounding rock under different spacing was discussed based on strength reduction method, and the reasonable tunnel spacing was determined. Results manifested that: 1) as for different excavation sequences, excavating the left tunnel first was superior to excavating the right tunnel first; 2) with the expansion of tunnel spacing, the displacement of surrounding rock of the left tunnel decreased continuously, while that of the right tunnel declined first and then increased; 3) also with the elongation of tunnel spacing, the horizontal displacement along the vertical center line of the intermediate rock column dropped gradually, whereas the vertical stress first reduced and then rose; 4) When the excavation of the right tunnel is completed, the bending moment at the right arch waist in the initial support of the two tunnels was obviously greater than that at the arch top, while the axial force was to the opposite; 5) with the increase of tunnel spacing, the plastic penetration degree of the intermediate rock column gradually weakened, the safety coefficient constantly improved, and the equivalent plastic strain gradually decayed. The research findings provide basis for the design and construction of small-spacing tunnel under unsymmetrical loading.