The current carrying capacity of the overhead transmission lines is lowered due to the conservative value of the boundary conditions.We propose an approach to calculate the carrying capacity based on K-Means algorithm,with purpose to fully exploit the carrying capacity of the overhead transmission lines.Meteorological data of Guangzhou in recent 10 years were summarized to analyze the variation of monthly ambient temperature and wind speed,based on which the K-Means algorithm was employed to rate the 12 months into several time periods,then appropriate boundary conditions were given for each time period.The Morgan formula was used to simulate the current carrying capacity under the new boundary conditions.Simulation results showed that the current carrying capacity can be effectively increased on the premise of ensuring safe operation of overhead transmission lines.