A method based on slope stitching for measurement of a large off-axis parabolic trough collector is proposed and applied to the surface shape reconstructed from the gradient data acquired by using the reverse Hartmann test. The entire reflector is divided into three sections with overlapping zones along the concentration direction. A mathematical model for the slope stitching algorithm is developed. An improved reconstruction method combining Zernike slope polynomials iterative fitting with the Southwell integration algorithm is utilized to recover the real three-dimensional (3D) shape of the collector. The efficiency and validity of the improved reconstruction method and the stitching algorithm are experimentally verified.