The invention discloses a coal and gas outburst testing device with a supplemental gas source and a method. The testing device comprises a coal sample tank, wherein one side of the coal sample tank is connected with a gas supplementing bag, the gas supplementing bag is connected to an air compressor, an opening is formed in the other side of the coal sample tank, a tempered glass plate is arranged on the inner side of the opening, and a pipeline is arranged on the outer side of the opening; a striking unit is arranged outside the pipeline, one end of the striking unit is fixed to the outer side of the pipeline, and a striking hammer is arranged at the other end and attached to the tempered glass plate; a plurality of pressure sensors and a plurality of photoelectric sensors are arranged at intervals on the inner wall of the pipeline, a light source is arranged in a position, corresponding to each photoelectric sensor, outside the pipeline, and a transparent panel is arranged between the light sources and the photoelectric sensors to allow the light sources to penetrate. Gas is continuously supplemented into the coal sample tank by the gas supplementing bag, the coal and gas outburst disaster occurring process is more truly simulated, pressure of gas in the pipeline can be measured by the pressure sensor, the moving state of outburst gas in the pipeline is reflected, and the moving position of outburst coal flow can be measured by the photoelectric sensors and the light sources.