Abstract: The present invention discloses an online fault detection device for train wheels, including: base rails 1, working rails 2, guard rails 3, and transducer detectors 4. A section is cut off from each of the symmetrically arranged base rails 1; a section of the working rail 2 transitions into and is attached to an outer side of the base rail 1 for bearing a train wheel; a section of the guard rail 3 transitions into and is attached to a position near an inner side of the base rail 1 for preventing a wheel rim from derailment; transducer detectors 4 and sensors thereof are placed in the space between working rail and guard rail; and the transducer detectors 4 perform automated detection by connecting to a data collection and processing system such as an external computer. With the present invention, good detection effects, low cost, and quick detection of a fault can be achieved.