In order to realize the precise measurement of flatness for large diameter narrow zone annular plane, a novel method based on optics and image processing using inner focus collimator and precision rotary table is proposed. Firstly, the image of point light source is projected above the annular plane using inner focus collimator. And the image point is driven by precision rotary table to draw a circle. The circle plane is the reference. The point light source is imaging on the CCD. Then the distance between annular plane and reference plane is measured. Next, the testing data is analyzed by means of Fourier harmonic analysis method. The influence of constant item and the first item is removed. Then the flatness of the annular plane can be calculated by the data processing. The basic principle of the testing method is analyzed. An annular workpiece with the out-diameter of 5 m and the inner-diameter of 4.4 m and the height of 0.5 m is tested with this method. The flatness is 0.285 mm, and the repeatability is 0.009 3 mm. Finally, the measurement uncertainty of this method is analyzed, and the composed standard uncertainty is 0.007 4 mm.