Abstract: In view of the fact of low accuracy of wellbore fluid temperature prediction and difficulty in identifying the position of the thief zone when the circulation loss occurs, a model of wellbore temperature field under lost circulation was established based on the analysis of wellbore heat transfer laws by comprehensively considering the influences of heat source items and variable mass flow on wellbore temperature. The reliability of this model was verified by field measured data, and the influences of the leakage rate and the thief zone on the temperature distribution laws of wellbore were analyzed. The numerical simulation results show that when compared with the model from Chen, the outputs from the new model are closer to the measured temperature, with the average relative error of 2.1%. The leakage rate imposes much greater influence on the bottom hole fluid temperature than the wellhead fluid temperature. In addition, when the leakage occurs in the upper open hole section, there is an inflection point on the temperature gradient distribution curve of the annulus fluids, and the position of the point is consistent with that of thief zone. The research results suggest that the model can accurately predict wellbore temperature distribution under lost circulation conditions, and the position of the thief zone can be determined in the field according to the annulus temperature gradient distribution curve.