Abstract: Based on the principle of polarizing modulation,a new method of phase retardation measurement and the experimental system are given.Investigative purpose is how to rise metrical precision and to rise metrical range.Measured result is metrical range from ultraviolet to near infrared and can measure other phase retardation.It is especially powerful for calibrating quarte-wave-plate and half-wave-plate,the metrical precision is less than 0.1%.