In order to achieve accurate positioning measurement, high precision star camera is the key equipment of the space mapping or reconnaissance camera. The difficulties of optical system design about star camera with long focal length are analysized. The new structure type of dual Gauss transited to telephoto is adopted, a subsecond star camera optical system is designed with focal length of 200 mm, relative aperture of F/2, angel of 7.5°×7.5°, spectrum range of 500~800 nm. Due to the characteristics of stellar temperature in large range, the color correction of optical system is realized without the use of special dispersion glass, the consistency of centroid position is achieved under different color temperatures of stellar. In order to compensate thermal drifting, negative temperature coefficient of refract index of the crown glass is adopted, by which sub- second precision of athermalization is come true. It is shown that, the distortion of the optical system is better than 0.003%, the lateral color aberration is less than 1.5 μm, the accuracy of centroid position is better than 0.2″ in the range of 2600 K~9800 K . Between 0 ℃ and 30 ℃, the change of focal length is less than 5.1 μm, the accuracy of centroid position is better than 0.4″, except the 1 field.