Before uncovering an outburst coal seam in crosscut and shaft, it is necessary to detect the effect of outburst prevention measure applied to the seam, which is a key step and the last link for ensuring the safety of coal miners. However, in the working face where outburst prevention measures have been taken, the large number of densely arranged discharge boreholes, as well as the difficulty in sealing pressure-measuring boreholes, makes it extremely hard to accurately detect the effect of the measure. In order to solve this problem, this paper studied the relationship between the initial rate of gas emission from the borehole (IRGEB) and the outburst danger through carrying out experiments on a self-designed oblong simulation device of outburst coal seam and a detection device of IRGEB. In addition, it proposed a method of testing the prevention effect using the initial volume of gas emission from the borehole (IVGEB) per unit length which actually refers to the time integral of IRGEB per meter. The experimental results show that IVGEB grows with the increase of outburst danger, but its critical value varies with the types of coal seams and gasses. As a result, it may take over 20 days to get the critical value of outburst detection of a specific coal seam. For this reason, this paper presented a simplified method of determining the critical value of IVGEB per unit length. When this method is applied, the total time taken up can be enormously reduced to shorter than 4 h just by drilling another borehole for measuring the IVGEB. The field application results indicate that the method can quickly detect whether outburst danger has been locally eliminated. Hence, it can be regarded as a very applicable method for the rapid detection of outburst prevention effect before coal uncovering in crosscut and shaft.