Abstract: Rapidly increasing electricity demand and access of high-penetration renewable energy generation have put great pressure on the existing transmission system. Dynamic line rating technology is an economic and effective way to improve transmission capacity of transmission lines. However, affected by accuracy of data acquisition, time-varying characteristics of calculation model parameters, etc., existing dynamic line rating systems still have some problems in practical applications, such as insufficient calculation accuracy and low reliability. To reduce the influence of data acquisition quality and ampacity model accuracy on the reliability of dynamic line rating technology, a correction method of ampacity and sag calculation model based on real-time closedloop calibration is proposed and constructed. Furthermore, combined with the numerical weather forecasting, a dynamic evaluation scheme of transmission line ampacity limit is proposed. Based on the dynamic calibration of the ampacity and sag calculation model, the rolling correction of the real-time and predicted ampacity limit is realized. Finally, the correctness and feasibility of the model calibration method and the dynamic evaluation scheme of the ampacity limit are verified through an actual engineering case.