The calculation process of swing angle measurement of drag parachute in rocket sled-drag parachute test is complicated and the error of calculation result is large, in this paper, a method of measuring the swing angle of drag parachute with monocular camera is proposed, which combines Photogrammetry and Spatial Analytic Geometry. The monocular camera is installed in the rocket sled compartment, and the camera range covers the motion range of the drag parachute, the swing angle of the main parachute rope is taken as the swing angle of the parachute. The source of error in the measurement of the swing angle of the drag parachute is analyzed, and the radial distortion and tangential distortion are corrected for the image point coordinates of the main rope of the drag parachute. The spatial coordinates of the center of the lens are known, the spatial coordinates of the image point of the main paracord after distortion correction are calculated, and the spatial straight line between the center of the lens and the image point of the main paracord is obtained. According to the collinear condition equation, the space straight line passes through the spatial coordinates of the node of the main paracord, and the movement path of the main paracord is a sphere with the radius of the paracord, the pitch and horizontal swing angles of the drag parachute can be calculated by solving the intersection point of the space linear equation and the sphere.