Abstract: One laser oscillating mode would split into two with an optical wave plate being inserted in the resonator. The frequency difference of splitting modes can be measured to obtain the wave plate phase retardation. Based on the principle, a measuring instrument for wave plate was designed with optical path along vertical direction, which could choose certain measuring method according to oscillating modes altered by frequency difference. The half-cavity laser, the detection of laser intensity and frequency difference, the controlling program were described in detail. In order to automatically measure wave plate with high precision, the instrument utilized the equal intensity point of two orthogonally polarized lights as working point, and compensated the systematic errors from initial phase retardation and slightly tilted wave plate. In testing experiments, the instrument can automatically discriminate and measure arbitrary retardance wave plates. The measurements show standard deviation of about 0.01 and uncertainty of 0.03 (less than /10 000) as to a multi-order quartz wave plate. The instrument has traceability and only the frequency difference of laser modes is acquired in measurement.