The invention relates to a real time measuring device and a method thereof for the orientation of a fast axis of a quarter wave plate. The device consists of a collimated light source, a polarizer, a standard quarter wave plate, a diffraction light splitting element, a focusing lens, a polaroid analyzer array, a photodetector array, an amplifying circuit and a signal processing system. The measuring method includes as follows: the parallel light beams exited by the collimated light source after passing through the polarizer and the standard quarter wave plate form a circularly polarized light; the circularly polarized light after passing through the quarter wave plate to be detected is split by the diffraction light splitting element; a plurality of sub-light beams with equal intensity are focused on the polaroid analyzer array by the focusing lens to respectively generate polarization interference for forming interference light intensities with a certain displacement in turn which are received by the photodetector array; a telecommunication signal outputted by the photodetector array after passing through the amplifying circuit is processed by the signal processing system, thus being capable of obtaining the orientation angle of the fast axis of the quarter wave plate to be detected in real time.