Fault diagnosis of transmission line is an important part in the construction of smart grid. Aiming at the problems of low data utilization and single upload of transmission lines, this paper applies S-transform and continuous hidden Markov model to fault diagnosis of transmission lines, uses S-transform to extract fault features of transmission lines, establishes continuous hidden Markov model through fault features, and realizes fault phase selection.The validity of the method is verified by simulation under different working conditions, the simulation results show that the method is not affected by the change of fault conditions, and can correctly identify the fault line under high noise. This study provides a reference for the development of transmission line fault diagnosis in China.