Abstract: By considering the internal and external factors influencing power transmission line running state,a running state and its risk evaluation method based on Markov chain model is proposed.First of all,the initial failure probability caused by external factors is quantified using the Markov chain extrapolated transmission line running state.Furthermore,by referring to the system power flow,the actual failure probability of line is calculated.Secondly,the system operation constraints are considered,the indices of fault severity and their weights based on analytic hierarchy process of an integrated index are defined so that theimpact"of fault on the system is quantified by these indices.Finally,the running state is evaluated based on risk assessment thought by combining the probability and the quantified impact.Simulation results show that owing to the overall consideration of both influencing factors and covering more risk factors,the method has a fairly good ability to recognize a line working at a high risk and likely to damage the system should fault occur.It has bypassed the difficult due to missed information,which may be of reference value to transmission line condition based maintenance research.