Abstract: The stress distribution characteristics of adjacent horizontal parallel tunnels are revealed by comprehensively utilizing the complex function theory, analytic continuation method and Schwarz alternating method. Then a mechanical model of adjacent horizontal parallel tunnels is constructed in combination with the D-P yield criterion which considers the effect of intermediate principal stress. The concept of connected radius of the tunnel plastic zone is proposed, the solving equation is established and its correctness is verified by numerical simulation. The critical distance of the plastic zone connected between tunnels is considered as a reasonable spacing, which has a better agreement with the spacing of tunnels when the displacement and settlement of surrounding rock do not basically change with the spacing of tunnels calculated by numerical simulation software FLAC3D, thus demonstrating its feasibility as the reasonable spacing of adjacent horizontal parallel tunnels.