Abstract: For working conditions of small pump capacity and low pressure of directional drilling in coal mine underground, a mine-used mud pulse wireless measurement while drilling (MWD) device was developed, and technique requirements, structure composition and application conditions of the MWD device were analyzed. Design scheme of signal generator in the MWD device, namely signal generator for mine-used mud pulse wireless MWD, was focused on. Structure design of explosion-proof driving section and its solenoid valve in the signal generator as well as pulse generator, and design of liquid passageway control valve and its valve head parameters in the pulse generator were introduced. The field test results show that the signal generator can generate mud pressure pulse signals stably and reliably with the maximum signal transmission depth of 2 570 m and low mud pressure pulse attenuation rate of 5.8%/100 m, which has ability to apply to deeper boreholes.