Extreme meteorological events such as lightning and ice disaster are the main reason of transmission line fault. Overhead ground wire is of lightning protection effect in summer and is easier to be iced than conductor in winter, and it directly affects the reliability of transmission line. It is necessary to study the effects of ground wire on transmission line reliability and the corresponding analysis method. This paper proposes a dynamic evaluation method of transmission line reliability which considers the effects of ground wire. ‘Month’ is taken as time scale rather than ‘year’ and the monthly failure rates of ground wire and conductor are respectively statistically calculated and fitted. The effects of ground wire status both in summer and winter on the reliability of transmission line are analyzed respectively, and the corresponding correction coefficients of conductor failure rate are introduced. Total probability formula is adopted to calculate the outage probability of transmission line, and the monthly outage probability function of whole year is obtained, which is related to both of the failure rate of ground wire and the failure rate of conductor. Finally, a specific sample is used to explain the method and the feasibility is verified.