As an important part of the smart transmission line， dynamic line rating can improve the utilization of transmission equipment and help the operator to know about the current operation status of transmission line. In order to ensure the reliability and safety of the transmission line， a method based on Markov chain Monte Carlo（MCMC） is proposed to assess the operation risk of transmission line using dynamic line rating technology. The main principle of this method can be stated as follows. First， the random sequence of micro-climate monitoring parameters posterior distribution is used to create the wind direction， wind speed and ambient temperature probability distribution model. Secondly， Monte Carlo（MC） simulation is used to predict the distribution of conductor temperature with climate models and the line load current model. Finally， the operation risk indices of transmission line are calculated using the conductor temperature distribution. Typical summer and winter monitoring data are used to analyze the reliability of dynamic rating， the result showing that the operation risk is within the security range and meets the requirements of grid operation and dispatching engineering.