The invention discloses a method and a device for quickly determining a coal seam gas parameter for an underground coal mine, and belongs to methods and devices for determining coal seam gas parameters. The method comprises: establishing a database about a gas parameter and a desorption curve by utilizing a one-to-one correspondence, determined by a lab, between the coal seam gas parameter and the desorption curve; matching underground field desorption data with the desorption curve in the database to acquire the coal seam gas parameter thereof. A coal seam gas parameter quick determinator of the determination device is provided with a flow meter, a single-chip computer, a display, a data storage device, a lithium battery power supply, and an intrinsic safety control circuit, and is connected with a coal sample tank through a gas guide pipe to determine values of coal seam gas pressure, gas content and drillings gas desorption indexes (K1 and K2). The advantages are as follows. The method can determine multiple coal seam gas parameters simultaneously, the needed time is short, the workload is small, the manufacturing cost is low, the operation is simple, the determination result is high in reliability, a judgment index is provided for outburst coal seam danger evaluation, and the method is of great significance for the development of an outburst prevention technology for a workface.