Abstract: During drilling the long open-hole section of extended-reach wells, wellbore is sensitive to complex and variable constraints; thus, how to accurately predict the operation limits of pipe strings is vital for safe and efficient operations. Considering the timeliness of measured data, a section-based real-time friction coefficient inversion method was developed to enable the real-time monitoring of borehole conditions during drilling. Moreover, a real-time prediction method of pipe string operation limits was established by integrating the prediction model of pipe string operation limits with the real-time friction coefficient inversion method. Finally, the concept of the pipe string operation safety factor was introduced, and a real-time evaluation method of pipe string operation risks based on the string operation limits was established. The case study shows that the proposed method can predict the pipe string operation limits in the drilling process of extended-reach wells in a real-time manner, identify the abnormal well sections, quantitatively evaluate the risk level of string operations, and clarify the main factors constraining the operation limit of pipe strings in a real-time manner. The proposed method provides a technical support for risk identification and optimization of drilling.