A Thomson scattering signal collective optic system is designed for Thomson scattering measurements on Shenguang-Ⅲ facility. The optical system is composed of two coupled Cassegrain systems, and has an intermediate image plane. This configuration makes the optical path length long enough to transmit the scattering signal to the camera outside the target chamber. The catadioptric system makes it easier to balance chromatic aberration as there are few ultraviolet materials available. The corrector lens which has a long focal length is replaced by two faster corrector lenses, which make the fabrications and measurements easier. The obscuration ratio is reduced by choosing a proper intermediate magnification. The two refraction lenses beside the intermediate image plane help to couple the two systems well. A reticle is set at the intermediate image plane for system aiming and can be removed before event recording. This system has the total length of more than 5 m, with the maximal elements diameter of 190 mm. The field of view is 2 mm, the magnification is 1.5, and the object space F-number is 10, and the resolution is better than 20 μm.