Firstly， based on the explicit analytic expression of conductor's resistance and temperature， an algorithm based on linear least squares state estimation is applied to determine the conductor resistance and hence obtain line temperature using information from phasor measurement unit（PMU）. Subsequently， the law of error propagation and probability expression is deduced when direct measurements are transformed into indirect measurements. Finally， with correlated measurement error， the probability distribution of the performance index is discussed in detail. The result shows that the traditional way for identifying bad measurement is still valid. Therefore， the tracking estimation of transmission line temperature is established， and the idea of soft dynamic thermal rating（DTR） is fulfilled. The method proposed provides a simple way to excavate the potential transmission capability， which shows a good practical prospect.