The invention provides an automatic positioning mechanism of a steel rail. Two sides of the steel rail are clamped by using a bearing 5, and the surface hardness of the bearing is high, therefore, the abrasion resistance between the bearing and the steel rail is improved; one side of the bearing is positioned, and the other side of the bearing can be automatically adjusted on a connecting rod 4 and always clamps the steel rail; the connecting rod is rotated by taking an intermediate shaft 8 as a pivot and a spherical gasket 3 as an adjusting point to realize any angle within a small range; and a disc spring 2 is used for providing an automatic clamping force, and the excellent positioning performance of the bearing on the steel rail can be ensured after the disc spring is adjusted, and meanwhile, the bearing can freely roll and run. The automatic positioning mechanism is mainly used for automatically positioning the device on the steel rail in a gapless way.