Abstract: Sweeping angle and rotating speed are important parameters of a parachute-ball ro-tator in wind tunnel measurement experiment as well as a necessary quantitative dynamic meas-urement test. The principle of the orthogonal projection method is to use a two-way orthogonal parallel light source of a given cross-section area to project the rotator into two orthogonal screens, two high-speed cameras synchronically record the projecting images. The image process-ing software automatically calculates the angularity of two-way image sequence, sequentially pho-tographs every moment's spatial sweep angle, and according to the sequential angularity, the in-stantaneous rotating speed and average rotating speed of all cycle can be calculated. Applying the orthogonal projection method, we have successfully measured different types of parachute-ball ro-tator in a vertical wind tunnel in Xi'an and obtained good results. The static angle measurement accuracy is 0.18°, the dynamic sweeping angle measurement accuracy is limited in 0.25°. This pa-per describes the principle and structure of the orthogonal projection method as well as the appli-cation of the image processing system, and the wind tunnel measurement results were also given.