In order to evaluate the blasting effect of high temperature and high pressure gas and determine the optimal initiation pressure, a crack identification method based on threshold filtering algorithm was designed,along with a crack parameter evaluation method combined with entropy weight method and box dimension. In this method, MATLAB software is used to identify cracks using thre-shold filtering algorithm, quantitative characteristics such as crack perimeter, fracture degree, crack width, and box dimension are obtained based on binary diagrams, and the crack sensitivity index is determined by entropy weight method, and the complexity of analysis is combined with box dimension to evaluate the blasting effect and determine the optimal initiation pressure range. The results show that the growth rate of crack width decreased from 62.2% to 47.6%, and the growth rate of fracture degree decrease from 47.6% to 13.2%. Furthermore, the increase of crack complexity gradually diminished, indicating that the utilization rate of blasting efficiency gradually decreases, and the initiation pressure range corresponding to the optimal blasting effect under the premise of ensuring energy utilization is from 120 MPa to 150 MPa.