The safety classification of transmission line crossing high speed railway was discussed. The target reliability index for the design of tower members and components in transmission line crossing high speed railway were proposed, as well as the relationship between the reliability index and the important coefficient and safety factor in the current design code of transmission line. The research results show that it can be divided into three security levels for the transmission line crossing high speed railway: in level 1, the reliability index should be more than 3.7 for tower members and components in transmission line； in level 2, the reliability index of tower members and components should be more than 3.2, and the reliability indexes in level 1 and 2 both should be more than the specified benchmark of reliability index； in level 3, the reliability index of tower members and components should not be less 0.5 than the specified benchmark of reliability index. There is a linear relationship between the reliability index and the important coefficients of tower members or the safety factors of insulators, fittings and ground wires in the current design code of transmission line.