Based on rigorous coupled wave theory, the relationship between diffraction property of rectangular phase gratings in resonant region and grating period, duty cycle, thickness is analyzed. An achromatic quarter wave grating for 400～800 nm is designed. In the broadband, the grating has smooth efficiency and phase retardation. Specially, the phase retardation could maintain about 90°, and the maximum retardation deviation is less than 9%. Its performance is even better than some normal binary achromatic wave plates. With the analysis of fabrication error, it is noted that thickness error and duty cycle error affect the grating′s performance greatly.