Abstract: In this paper, the drill string bore is regarded as an irregular lossy cylindrical waveguide, in order to obtain the best frequency point, the attenuation law and the effective transmission distance of the microwave channel. The best transmission frequency point of microwave is calculated by the electromagnetic wave coupling theory, and the microwave mode in the channel is analyzed. Furthermore, the channel model is established by microwave transmission equivalent circuit method and the signal attenuation rule of drill string bore microwave channel is established. In addition, in view of the ultra-long drill string channel, this paper proposes a simplified analysis method of effective transmission distance using average loss coefficient of unit length. The test results show that the pipe bore of drill string has several inner diameter gradual change sections and sudden change points along the axial direction, and there are a lot of reflection processes in the microwave transmission, therefore the channel impedance changes is the main factors affecting the transmission quality. The attenuation model established in this paper is more accurate than the model considering only the eddy current loss of surface resistance, which is more helpful for on-site guidance.