In a single slit Fraunhofer diffraction device, the dissymmetry distributing diffraction pattern can be seen on the screen by turning one edge of the single slit and fixing another. In order to ascertain the accurate position of diffraction fringe according to the math expressions, the least square method is used to fit the curves of the intensity distribution. The relation between the displacement of the turning edge in two perpendicular directions of turning plane and the positions of diffraction dark fringes can be determined and it can be used to calculate the small rotating angle. Experiments show the new method can be applied to measure small rotating angle accurately. The uncertainty of the rotating angle is 0. 8.