A real-time measurement method for retardation of eighth waveplate (EWP) independent of fast axis is presented. The collimated beam passes through the circular polarizer (CP) and the EWP to form a measuring beam. The beam is split to four diffractive beams by a two-dimensional orthogonal grating (2DOG). They are detected by a photodetector array (PA) after passing through a polarizer array (AA). The four light intensity signals can be used to achieve real-time measurement of retardation of EWP. The result is not affected by the fluctuation of the intensity of the laser. In experiments, an EWP whose nominal value of retardation is 45° with accuracy of λ/300 is measured. The mean and standard deviation of measurement results for retardation in different directions of the fast axis are 45.1° and 0.4°. They coincide with the nominal value and accuracy of the retardation, respectively. The usefulness of this method is verified.