To accomplish the sub-aperture stitching testing for large aperture flat mirror under considerable adjustment error, we establish a reasonable stitching algorithm and mathematical model based on iterative gradient algorithm. At the same time, relative program is written for stitching. Combined with engineering examples, we measure a F800mm flat mirror with a F600mm interferometer in stitching method. In the measurement, we accomplish the alignment between sub-apertures with targets and the stitching result is smooth. Experimental results show that stitching measurements of large aperture flat mirror could be well down with the this algorithm under considerable adjustment errors.