Many types displacement sensors used in active optics in many astronomical telescopes over the world ware describedand the measurement theory of different sensors were explained in this paper. Based on the displacement sensor’s specification of LAMOST, a test bed to check it which used dual-frequency laser interferometer was established. Some main parameters affect on the measurement accuracy of the test bed such as temperature, vibration as well as some mechanism characteristics of fixing devices were analyzed in detail, and correspond solutions were adopted. According to theoretic analysis, rectification method was brought forward to compensate the errors of sensor cause by tip-tilt, thermal shift, non-linearity, and combined with the test results of some types sensors, their advantages and disadvantages was concluded. At last it forecasted the most appropriate displacement will be used in future active optics based on the discussion above and the developing trend of large telescope.