Provided is an over-positioning grinding method of a hard and brittle pipe fitting with large length-to-diameter ratio, which belongs to the field of grinding technologies. The existing machining method cannot increase the grinding accuracy of the external circles of the hard and brittle pipe fittings nor ensure the coaxiality of the external circles and internal circles of the ground hard and brittle pipe fittings, which leads to inconsistent wall thickness of the ground hard and brittle pipe fittings. In the present disclosure, by using the over-positioning supporting of the rigid mandrel, the support plate and the guide wheel for the hard and brittle pipe fitting in a machining process, the guide wheel is driven to rotate forward to push the hard and brittle pipe fitting with large length-to-diameter ratio into the grinding area, and the guide wheel contacts and drives the hard and brittle pipe fitting while rotating reversely.