Theoretical analysis and numerical simulation on the error accumulation in sub-aperture stitching for flat optics show that the slope and piston difference of the surface figure of the reference flat in the overlap region lead to error accumulation. In order to improve the measurement accuracy of the stitching interferometer for large aperture flat optics, a simple and effective method is proposed to reduce the accumulated error of the sub-aperture stitching measurements. The 4th and 6th Zernike aberration are used to synthesize the virtual reference flat. Sub-apertures with the virtual reference surface figure removed are stitched together to obtain a full aperture surface figure. A flat mirror with aperture of 450 mm×60 mm is tested by 8 sub-apertures. The deviation between the stitching result and the test result of a Zygo 24 inch (600 mm) interferometer peak-valley (PV) value is reduced from λ/7 to λ/100 after removing the virtual reference surface figure. The flatness of the virtual reference surface is 0.02 λ (PV value), which is comparable to the real figure of the reference flat. The influence of removing of virtual reference surface on the measurement results of each sub-aperture can also be ignored. Experimental results show that the accumulated error is controlled effectively and the measurement accuracy is evidently improved by the proposed method.