The interferogram is usually obtained with a low unconstant S/N rate in engineering interferometry, which makes it very difficult to analyse the fringe pattern by using fringe thinning and fit method, and causes larger test error. The Gaussian threshold method introduced in this paper can overcome the disadvantages of some ordinary techniques widely used now. It is certified that this method has high accuracy and reliability by the analysis of computer simulated results and real tests. The principle and algorithm of the Gaussian threshold and the results of tests are presented in details.