A new method for measuring the retardation of waveplates in the infrared spectral region is derived by analyzing the light intensity variation with the azimuth angle. The phase retardations of waveplate are determined through reading out the maximum and minimum of light intensities and simple calculation. Based on this method, an instrument is set up to measure the retardations of the waveplates in the infrared range. With less devices, easy operation and sound repetition, measurement results with high precision are obtained. The effects on the measurement precision of the different sources, such as light source, polarizer and detector, etc., have been analyzed. It is found that the instrumental accuracy is related to the waveplate retardation, and their relation is plotted. From the plot it is found that the measurement precision is within 1% when the retardations of waveplates are more than 40°. The relative errors are measured to be 0.2% and 0.01% for the conventional quarter and half waveplates, respectively. The measurement precision of the system is almost constant in the visible and near-infrared range.