Objective　To evaluate the correlation between the measured value of bone mineral density(BMD) measured by quantitative CT(QCT) and fragile fracture of osteoporotic spine,so as to find the critical value scope of BMD for fragile fracture.Methods　80 postmenopausal women who hospitalized from June 2016 to March 2018 with single fragile fracture of the spine for the first time were enrolled in this study.The values of BMD of QCT of non-fractured vertebrae in T11~L4 was measured by 64-slice CT and compared with those of 80 asymptomatic postmenopausal women who volunteered to participate and met the inclusion criteria in the same period.Results　In the experimental group,63 cases(78.75%) had QCT bone mineral density in the range of 35~50 mg/cm3,69 cases(86.25%) had QCT bone mineral density below 50 mg/cm3,and only 11 cases(13.75%) had QCT bone mineral density above 50 mg/cm3.The average value was(43.63±7.54)mg/cm3.In the control group,53 cases(66.25%) had QCT bone mineral density in the range of 50~70mg/cm3,73 cases(91.25%) had QCT bone mineral density above 50 mg/cm3,and only 7 cases(8.75%) had QCT bone mineral density below 50 mg/cm3.The average value was(64.57±12.07)mg/cm3.The difference of the average value of QCT bone mineral density between the experimental group and the control group was statistically significant(t=13.158,P<0.001).Conclusion　Postmenopausal women with osteoporosis are prone to osteoporotic fragile fracture when their QCT bone mineral densities reduce to about 50 mg/cm3,and the QCT value of 50 mg/cm3 is likely to be the critical value of BMD for fragile fracture.