Disclosed are a measurement-while-drilling method and device for obtaining a coal seam gas parameter applicable to measurement of a coal seam gas/coal seam methane parameter while drilling and evaluation of gas drainage performance. The invention performs a measurement-while-drilling operation to obtain information about downhole gas and a coal seam, thereby realizing on-site measurement of a coal seam gas parameter while drilling. The measurement-while-drilling device for obtaining a coal seam gas parameter comprises a compartment (17), a pressure sensor (15), a temperature sensor (14), a flow sensor (4), an acoustic emission sensor (16), a monitoring and control module (11), a power module and a communication module. The device is installed between a drill bit (1) and a first drill pipe (20). The method comprises: in a drilling process, using the temperature sensor (14) to monitor, in real time, a temperature at a spot under measurement; when drilling stops, sealing a borehole to form a pressure measurement cavity, and using the pressure sensor (15) to monitor a gas pressure in real time; after completion of the pressure measurement, using the flow sensor (4) to monitor, in real time, an amount of gas emitted from the borehole; and performing comprehensive calculation to obtain a gas pressure, a gas content, and an amount of initially desorbed gas at the spot of the coal seam. The invention can perform on-site measurement while drilling and calculate to obtain a coal seam gas/coal seam methane parameter, can perform on-site measurement while drilling without sampling, and can use multiple parameters to perform synchronous measurement.