According to above analyses and considering the applicability under special conditions, a mathematical expression Equation 1) was put forward to represent the Accident triggered likelihood degree. The equation shows that the smaller the number m of the upwind release source and the sum of (the minimum angle between release source and horizontal axis), the smaller the Accident triggered likelihood degree of current ignition source. In addition, the larger the sum of distances between release source and ignition source, the smaller the Accident triggered likelihood degree of current ignition source.