For common methods of investigating the vortex motion evolution experiment in a towing tank, a cross-flow plane is usually illuminated by the laser sheet, and vortex motion photos taken by the CCD camera which is placed on one side of the end face of the towing tank are usually analyzed. However, this method only contributes to investigating vortex planar motion. For offering further insight into its spatial evolution process, a new method named Lagrangian section method is developed to extract and restructure the three-dimensional complex vortex system structure. The method is based on the Lagrangian theory to depict the flow field, combined with the image processing technology of Matlab software to transform vortex motion photos into digital ones. Owing to Galilean transformation, their spatial relative locations are determined by considering the trailer speed and the camera shooting interval. After they are arranged logically, the three-dimensional spatial structure of the vortex system is restructured well to depict the real physical structure. The new method can help people cognize the evolution phenomenon on three-dimensional complex vortex system directly and comprehend vortex interaction mechanism better. Compared to common research technique, it has the characteristics of high cost performance.