A new method, "self-guiding method", for ruling long diffraction gratings is advanced in this paper. The key point of this method is that the grating being ruled plays a role as a datum grating to lead a photoelectric controlled ruling engine to continue its ruling. The two main parts of this method, i.e. precision analysis and experiment devices are described in details. We have ruled several self-guiding samples of grating with one switch. The spectrum quality of them is as good as a general one, the actual resolution power of them with 50 mm width is about 90% of the theoretical value. If the traval of the engine is long enough^ a very long diffraction grating can be ruled by this method.