Abstract: The main blade girder was fractured after more than 800 hours of service. when the beam crack penetrated the wall thickness, the pressure found that there was no pressure difference warning, which caused the subsequent continuous expansion of the girder until the fracture accident. In the present work, macro and micro observation, and quantitative analysis on fracture surface were carried out, the load spectrum of the outfield aircraft is analyzed. The expansion rates of the girder at different stage are given and verified by simulation experiments. The extended life of the main blade girder was about 40 ups and downs, and the initiation life was 2097 ups and downs. The results of fracture quantification can make up for the defects of insufficient field monitoring, provide direction for troubleshooting, and provide services of life assessment and inspection cycle of main blade girders, so as to evaluate the damage tolerance of girders. when the crack penetrates the wall thickness pressure sensor, the damage tolerance period is at least 10 ups and downs.