A method for rapid measuring retardation of a quarter-wave plate based on simultaneous phase shifting technique is presented. The simultaneous phase shifting function is realized by an orthogonal grating, a diaphragm, an analyzer array, and a 4-quadrant detector. The intensities of the light beams from the four analyzers with different azimuths are measured simultaneously. The retardation of the quarter-wave plate is obtained through the four light intensity values. In this method, the major axis position of the quarter-wave plate need not be determined in advance. In addition, the measured result is free of the intensity fluctuation of light source. The feasibility of the method is verified by the experiments.