Abstract: For the application demand of high image quality collimator with large diameter, long focal length and wide field, an off-axis three-mirror configuration for the light collimator was proposed. Based on the coaxial three-mirror configuration theories, the computation of the initial structure of the system was dervied, and a method of optimizing the off-axis amount was put forward by using the ZPL language of zemax software. A wide field off-axis three-mirror light tubes was designed, with focal length of 10 m, F-number of 28.57. Results show that the modulation transfer function of this system is near the diffraction limit, within the field 21, the full field wave-front aberration is less than /200(=632.8 nm), the total length of the system is less than f'/3, providing a possibility design of long focal length, large diameter, and wide field of view light collimator.