A practical and scalable alignment automation system for muti-projector display wall was constructed based on the measurement of low-performance cameras. Taking the advantage of re-alignment theory the homography matrix was refined and used to achieve seamless alignment across different areas on the projection screen. The special pattern for feature extraction and the re-alignment idea furthest decreased the effect of the cameras- performances on the alignment precision. The system can automatically align the projectors under the conditions that there are overlapped view areas between adjacent projectors and cameras can observe them. It is shown in the experiments that the re-alignment method obviously shortens the calibration period and brings us sub-pixel alignment precision. The low-performance cameras, high efficient feature extraction method and sophisticated solution for homography make the cost of expanding the system low.