A new method for measuring the retardation of waveplates with high precision is brought forward. The measured waveplate is placed between polarizer and analyzer, and rotates uniformly under the control of stepper motor. The phase retardation can be calculated with the least square method fitting the curve of emergent light intensity variation with the azimuth angle of waveplate. Based on the analysis, an experimental system is established to measure the retardation. Four parts, including stability of the system, retardation range for measuring, nonlinearity of the detector, the main error sources of the system are analyzed. It is found that λ/2 waveplate cannot be measured in this system; higher stability can be obtained when the azimuth angle of analyzer ranges from -38° to 38° and the sampling interval is less than 10°; the detector has large second-order nonlinear effect; the measurement precision is mainly affected by the uncertainty of azimuth angle of waveplate and analyzer; repeatability of the system is within 0.1° except for the areas near 0\O, 180° and 360°. The measurement precision of the system is almost constant in visible range.