A method for simultaneous measurement of the retardance and the fast axis angle of quarter-wave plate using one photoelastic modulator is presented. A laser beam passes through a polarizer, a photoelastic modulator, the quarter-wave plate to be measured, and an analyzer to be detected. Before and after the quarter-wave plate is rotated $45°$ at any initial fast axis direction, two detection signals are obtained to resolve simultaneously the retardance and the fast axis angle. In experiments, a quarter-wave plate was measured with fast axis angles from $-89°$ to $90°$ . The average and the standard deviation of the retardances at different fast axis directions are respectively $89.50°$ and $0.17°$ . The maximum measurement deviation of the fast axis angle is $0.5°$ . The usefulness of the method is verified.