The invention discloses a wheel diameter measurement method for an urban rail train. The method comprises the following steps: at least three sensors Sn are arranged sequentially on the inner side of a track, wherein n is an identifier of the sensors, and n is larger than or equal to 3; coordinates of a to-be- measured wheel at a measurement point are obtained through the sensors; the coordinates are transformed to a geodetic coordinate system with the circle center of a standard wheel as the origin; effective data points are extracted under the geodetic coordinate system after transformation, part of data points of a rim are obtained, and a rim contour is established; the lowest rim point is extracted from the rim contour, and the wheel diameter is solved on the basis of the lowest rim point. The wheel diameter measurement method has the characteristics of simple measurement principle, high calculation speed and accurate measurement result.