Abstract: Geological guarantee technology based on directional borehole can realize the advanced detection and treat- ment of hidden hazards in underground coal mines. However,it is restricted by the existing wired MWD directional drilling equipment,because of many interference factors of signal transmission,high requirement for directional drilling tools,poor adaptability of stratum and other shortcomings,so it is necessary to develop the wireless mud plus MWD system. Combining with the construction characteristic of the near-horizontal directional borehole in underground coal mine and the working requirement under the conditions of small pump volume,low pressure and small diameter,the generation mechanism of mud pulse signal and the model of hydraulic channel are established. The characteristic of mud pulse signal about transmission speed,transmission intensity,attenuation law and transmission factors are ana- lyzed,and the pertinent treatment methods including the signal reflection and transmission,the interference from mud pump noise and bit rotation,and the blockage of fluid channel are given. The analytical results show that the mode of positive pulse is suitable for signal transmission in near-horizontal directional borehole in underground coal mine. The signal transmission speed is 1 366. 20-1 445. 11 m / s when water is used as flushing fluid. The signal pulse amplitude can be adjusted by changing the gap between the control valve of flow channel and the orifice. The attenuation coeffi- cient of signal transmission increases linearly with the increases of fluid flow rate,and decreases gradually with the in- creases of inner diameter of drill string,additionally,the attenuation coefficient is maintained at about 0. 126×10-3 m-1 when the signal transmission frequency is greater than 0. 2 Hz. The received signal intensity at orifice is 73． 74% -90． 71% of the transmission signal intensity when the depth of borehole is 1 500 m by common drill pipe in under- ground coal mine. The field test shows that the error rate of signal transmission in the mud pulse MWD system is 3． 2% ,the amplitude of actual pulse signal is less than theoretical calculation pulse signal,but the error is less than 10% ,the maximum transmission distance is 1 566 m with a deeper signal transmission capability.