Recently, the photomechanics employed to measure displacement, strain or stress has been developing rapidly. In photomechanics, the identification of interference fringe order and the density of interference fringe are attractive to the researchers in this field. It deals with the quantity analysis of the measured object, the accuracy and sensitivity of measurement and the real reflection about the whole nature of the measured value, etc. Moreover, the automatic'. identification of interference fringe is also the technical basis of automatic process of interference fringe pattern. At present, the identification and increase of interference fringe can be divided into three groups in which one is the new photocarrier methodc(1-2) , the second one is the optical heterodyning method and the third one is the changed parameter method presented here based on the paper(3). This paper presents the formulas, the physical sense and the applications about the changed parameter method.