Reliability assessment for selected paths is one of the focuses in path selection of transmission line planning and design process. Considering the disadvantages of conventional methods which adopt average annual failure rate, a transmission line path design method based on reliability analysis is proposed in this paper. Based on statistics for historical failure information of existing lines in the area where selected paths cross, failure rate of each month of selected paths is calculated in monthly scale and its monthly time behavior is fitted. Owing that the power supply output connected to the selected path may be also time-varying, the monthly time behavior of power supply output is then fitted. Finally, according to fitting functions of selected path failure rate and power supply output, the reliability level corresponding to the selected path can be reflected through calculating the expected energy loss of full year, which can provide important reference for comparing several selected paths and choosing the optimal path.