The invention discloses an absolute grating scale light source automatic assembling apparatus and assembling method, and belongs to the field of a grating scale assembling technology, for the purpose of solving the problem of automatic assembling of a light source of a grating scale. An indication raster and a photoelectric sensor are superposed with the optical axis of a light source; a signal extraction reference raster performs reciprocating movement in an x direction by taking a central axis as a center; a second trigger switch is disposed below the central axis and is disposed at the lower surface of the lower frame of the signal extraction reference raster; a first trigger switch is disposed at the left side of the second trigger switch, is distanced one third of the length of the signal extraction reference raster away from the central axis and is in parallel arrangement with the second trigger switch; the first trigger switch and the second trigger switch are respectively connected with a lower computer module; and light, after passing through the signal extraction reference raster and the indication raster, is incident into the photoelectric sensor, optical signals are converted into electric signals, the electric signals are transmitted to the lower computer module for signal conversion and processing, then the electric signals are transmitted to a host computer module, and the host computer module analyzes the signals and then emits an adjustment instruction.