In a coal mine tunnel,if the antenna beam is too wide,it is difficult to effectively suppress the multi-path fa- ding and to increase the coverage area. On the contrary,if the antenna beam is too narrow,the edge areas of the tunnel cannot be well covered. Based on the theory of multimode waveguide,it is discussed and concluded that the radiation characteristics of the ideal point source in the tunnel is the theoretical basis for the formulation of mine antenna beam index. Then the direction function and the half power beam width of the ideal point source in the underground roadway are derived. On this basis,and by the statistical calculations and simulations,the influences of the tunnel waveguide structure,the dielectric parameters of the tunnel walls, the carrier frequency and the propagation distance are dis- cussed. According to the analysis,and combining with the mathematical derivation and comparison,the optimum beam index of the mine antenna is given. The results show that in order to improve the wireless coverage significantly,the mine antenna should radiate flat π / 2-mean Gaussian beam. Meanwhile,the half power beam width must be at least narrower than that of the ideal point source when its radiated fields propagate to the break point of section I and II in the tunnel.