With the increase in coal mining depth, the ground stress and gas pressure of the coal seams increase gradually, the permeability of the coal seams drastically decreases, the rockbursts, coal and gas outbursts and other power disasters of the coal seams are becoming increasingly serious, and especially extremely soft coal seams that have been subject to tectonic destruction lead to the increase of the danger of the coal dust disaster. At present, the coal seam water injection techniques are frequently utilized in China to prevent and control rockbursts, coal and gas outburst, coal dust and other disasters; different water injection pressures and modes are used depending on different disaster characteristics, these techniques include low and medium pressure water injection wetting, high pressure water injection, ultrahigh pressure water injection fracturing and other water injection techniques with different pressures and further comprise static pressure water injection, dynamic pressure water injection (pulse fracturing) and other water injection techniques in different modes. After various water injection measures are taken on site, the effectiveness of coal seam water injection is often investigated, and especially the wetted perimeter of the coal seam water injection needs to be determined in the field of dust prevention and control. At present, for the existing investigation method, the moisture content is determined mainly by means of sampling the coal on site after water injection so as to determine the wetted perimeter of coal seam water injection, the moisture in the coal fissure in the test point area gets lost in advance owing to the influence of the investigation boreholes, resulting in large test errors of coal moisture content when this method is used to test the moisture content, and for this method, the samples are taken and tested after completion of water injection, resulting in failure to reflect the dynamic penetration process of coal seam water injection, thus leading to failure to provide adequate scientific reference for the optimization of coal seam water injection parameters.