Abstract: The geometric parameters of similar model for small and medium sized coal and gas outburst were studied by theoretical analysis, based on the statistical characteristics. The results show that the outburst hole is simplified as an ellipsoid and the opening is simplified as a round. According to the statistical geometrical parameters of coal and gas outburst, and the minimum statistical relationships of 2 times between outburst coal volume and the outburst hole volume, gas emission amount and gas content of ton coal, the geometrical parameters of the medium and small coal and the gas outburst holes are deduced, the depth of hole is 5 m, width is 4 m, and orifice diameter of hole is 0.68 m to 1.94 m, and the depth of coal and outburst prototype is 12.3 m, width is 6.5 m, and orifice diameter of hole is 0.68 m; in order to guarantee the movement similarity of outburst coal, according to the relationship among statistical data of outburst coal size range, hole orifice size and height of tunnel, the minimum size of hole opening was calculated. Selecting 10 times of coal particle size 5 mm as the minimum outburst hole size, the maximum geometric similarity ratio between coal and gas outburst model and prototype was deduced as 13.6. So, the minimum geometrical parameters, the depth, the width and the hole opening diameter for coal and outburst similar model, are 0.9 m, 0.48 m, 0.05 m, respectively.