The invention provides a switch tube open-circuit fault diagnosis and positioning method of a high-speed rail traction system single-phase three-level rectifier, and belongs to the field of fault diagnosis. The method comprises the following steps of establishing a hybrid logic dynamic model, establishing a novel self-adaptive sliding mode observer, providing a primary diagnosis self-adaptive threshold, performing primary fault diagnosis, providing a secondary diagnosis self-adaptive threshold, performing secondary fault diagnosis and positioning, providing a third diagnosis self-adaptive threshold, and performing third fault diagnosis and positioning. According to a self-adaptive sliding mode observer provided by the invention, a system does not need to provide a very accurate dynamic model, only a sliding mode surface needs to be reasonably designed by utilizing a tracking error of a track, and the approaching rate can be adaptively switched, so that the approaching speed is accelerated, buffeting of sliding mode motion is reduced, and the diagnosis accuracy is improved; according to the method, fault diagnosis and positioning are carried out on a switch tube by comparing the relation between the residual characteristic quantity and the self-adaptive threshold value.