In this paper a super high precition displacement measuring device with resolution better than 0.001um is introduced. We have improved Michelson interferometer using He-Ne laser and co-optical path with double optical beams. So high stability (<0.0015um/h), high repeatability (<0.001um) and 2A sensitivity are achieved. A reference mirror excited by a osillating source forms two trains of sinusoidal waves, after being processed electronically, a subdivision of X /800 is obtained i.e. equivalent value is 0.00079um/pulse. Through special calibrating method we find the accuracy of fringe subdivision is ±0.0025um. The instrument is used to check the accuracy of micro displacement measuring comparator of which the sensitivity is 0.01um--0.001um.