Abstract: A simple inertial or Doppler navigation has a common defect that position errors are always increasing with flight time or distance, therefore an external reference system is necessary. In order to meet the needs of high accuracy and reliability for modern vehicle navigation, modern navigation sys- tems show a tendency to be a hybrid type. This paper presents an astro-inertial hybrid navigation system, which is physically more rational and accuracy and comes into particular use in various modern vehicles. A star tracker in astronavigation is an essential facility to obtain the measurement information of the hybrid system. This paper summarized the procedure in research and development of an automatic star tracker designed by authors and the measures to solve the principal technical key problems, and also presents all related formulae. Experimental results have proved that our design is reasonable and has attained the expected characteristics, according to which the designed star tracker on the ground is capable of tracking stars up to brightness of second class at night and the brightest stars under the background of certain brightness in daytime.