Aiming at the measurement needs of large-aperture optical elements with ultra-large dynamic range, a sub-aperture stitching testing method based on optical deflectometry was proposed. According to the surface feature, the sub-apertures were divided and sequentially measured with the proposed fringe-illumination deflectometric testing system. Based on the slope data measured with actual testing system and ray-tracing result in the system model, the tested surface in each sub-aperture could be reconstructed with high accuracy and be stitched for full-field testing. Compared with the interferometric testing method, the optical deflectometry testing was larger in dynamic range and field of view, which could greatly reduce the number of subapertures required, thus greatly improving the measurement efficiency. Additionally, a weighted-fusion algorithm based on overlapped regions was proposed to obtain the smooth stitching result. To demonstrate the feasibility of the proposed method, both the numerical analysis and experimental verification were carried out. The high accuracy and large dynamic range were validated in the reflective lampshade testing. The result shows that the stitched surface obtained by the proposed method is consistent and smooth, and its surface deviation RMS compared with the full-aperture measurement result is 0.0957 µm, which is smaller than microns. The proposed method is high in measurement accuracy, large in dynamic range and also simple in system configuration, providing an effective and feasible testing method for various optical elements with complex reflective surfaces.