The traction system of high-speed maglev train is driven by the long stator linear synchronous motor (LSLSM). In order to realize sensorless operation at high speed, the observation of extended back-EMF is a key technology. Considering the complexity of the mathematical model of the LSLSM in the double feed mode, a new sliding mode observer model is proposed in this paper. By considering the converters at both ends as a whole in the dq-coordinate system, and performing closed-loop control of the stator current and the circulating current of the converter separately, the motor stator current is reselected as the observer state variable, which significantly simplifies the observer model. Finally, sensorless control is realized by the quadrature phase locking technique. Simulation results verify the effectiveness of the algorithm.