Abstract: Transmission line temperature is an important indicator to evaluate the capacity of transmission lines, which can be used as the basis for dynamic capacity expansion and guide the real-time scheduling decision of power grid. Based on the measurement and heat balance equations, a real-time estimation model of transmission line temperature is proposed. The line temperature is taken as a state variable and the dynamic heat balance equation is introduced as a pseudo measurement. The heat balance equation is discretized, and then the alternating approach and simultaneous approach are used to solve the differential-algebraic equations. Considering the complexity of meteorological condition along the long transmission line, a more refined method for segmented temperature estimation of transmission lines is proposed by introducing virtual nodes into the segmented transmission lines. A two-port network equation is used to calculate the measurement of virtual nodes. The effectiveness of the proposed algorithm is verified by the improved IEEE 5-bus, IEEE 39-bus systems and a real line. The results show that the proposed algorithm can adapt to various scenarios and estimate line temperatures quickly. With the segmentation of lines, it is possible to accurately estimate the real-time temperature trajectory of each line section, which shows strong practicality.