The fault recording data collected by the devices of both sides of transmission line, owing to the error in the sub clock, is asynchronous.The results of subsequent analysis based on the unprocessed fault recording data, especially the fault location, can not meet the requirement of accuracy, especially the fault location, failing to provide guidance for fault resolution.To make full use of fault recording data, time synchronization of recording data is very necessary.Firstly, three methods currently commonly used in engineering are shown, at the same time, the principle and limitations of every method are discussed.Secondly, the transmission line with lumped parameters model is established, a method based on least squares, using the parameters and recording data of the transmission, is proposed.Finally, the simulative results demonstrate that the time synchronization method based on least squares can achieve high accuracy in fault location, and improve reliability of subsequent analysis results.