Objective To find out the critical value of necrosis index in femoral head collapse through correlation between necrosis index and magnetic resonance image (MRI) staging of ischemic necrosis of femoral head, so as to provide a more reliable reference standard for prediction of femoral head collapse with MRI (to predict whether ischemic necrotic collapse will happen in patients with ischemic necrosis and collapse of femoral head). Methods A total of 70 cases of patients (100 hips) were enrolled in this study. The patients were diagnosed as ischemic necrosis of femoral head between June 2012 and February 2014 in our hospital with MRI images that could be definitely staged. MRI images of these 100 ischemic necrotic femoral heads were staged according to LennoxHungerford staging method. And necrosis index was calculated using the method proposed by Koo et al. A statistical analysis was carried out to evaluate their correlation and to determine the critical necrosis index at femoral head collapse. Results Necrosis indexes of ischemic necrotic femoral heads at stage I to IV were (0.31±0.08)(95%CI:0.29-0.34), (0.50±0.20)(95%CI:0.43-0.57), (0.62±0.18)(95%CI:0.53-0.71) and (0.83±0.23) (95%CI:0.70-0.95), respectively. Significance test showed that the statistically significant differences between femoral heads with different stages of ischemic necrosis in necrosis indexes (P＜0.05). HSD method showed statistically significant differences between any of the two groups in necrosis index (P＜0.05). There were only 3 hips in stage II (no collapse) had necrosis index＞53%, while each hip of stage III (began to collapse) had necrosis index＞53%. Conclusion Necrosis index of ischemic necrosis of femoral head is positively associated with necrosis staging, calculated according to MRI images, necrosis index of 53% obtained in statistical analysis can be taken as critical reference to predict collapse of femoral head necrosis. In the clinical treatment, necrosis index can be used as reference for predicting whether ischemic necrotic femoral head collapses.