Contact type interferometer is widely used equipment for testing relative length. It has been used to verify gauge block by metrology department because of its high accuracy. A computer-aided system based on it is introduced. There is a data acquisition system, including CCD and grabber etc., to detect and digitize interferograms instead of estimated by operator and avoids eyestrain. This system can analyze 2D interferogram automatically and quickly while both calibrate with monochromatic fringes and test with achromatic fringes. Many methods were used to analyze the 2D interferogram. It has been used by metrology department and its accuracy is better than 0.02 micrometers . Its resolution is better than the range per 1000 which is higher than the resolution estimated by operator. Different software has been developed to suit for different applications. One is the process-controlling software for verifying gauge block. The other is continuous- measuring software for testing tiny displacement about micron, such as measuring the responsive function of PZT.