Abstract: Aiming at the problem of severe signal attenuation and limited applicable depth of electromagnetic- while-drilling measurement in horizontal drilling in underground coal mine, the electric field components in layered lossy medium are solved with the propagation theory of electromagnetic wave in layered medium and the interpolation algorithm based on two-level discrete complex image method, and the current on drill pipe and electric field in coal seam are calculated by moment method of piecewise pluse functions and point matching, then the signal strength at pithead is obtained. Moreover, the 5-layer signal channel composed of basic roof, immediate roof, coal seam, immediate floor and the basic floor was built with the developed model. Then, the factors affecting signal strength were analyzed. The simulation results show that the signal strength increases first and then reaches saturation with the increase of the resistivity of coal seam and surrounding rock, while the coal seam has the most significant effect on the signal strength. Setting electrodes on the same side of the drill pipe, increasing the length of the lower drill pipe, reducing the contact impedance between the drill pipe joints and reducing the signal frequency are the effective methods to enhance the signal strength.