Abstract: The collimator is a commonly used optical precision instrument in optical laboratory, it simulates an infinite target by illuminating the target with the light source, which is the necessary equipment for assembly, adjustment and testing of optical system. A collimator with 200 mm aperture and 5 000 mm focal length was designed and off-axis two-reflective system structure was used, and the field angle was 0.35°. Both primary and secondary mirrors were aspherical and the secondary mirror was convex double-curved surface. The design value of system center field-of-view wave aberration was 1/62 λ, and that of edge field-of-view wave aberration was 1/21 λ. The inspection scheme of secondary mirror of this collimator was studied, and a simple and reliable front testing scheme for convex double-curved surface secondary mirror was proposed.