As an important trend in the future, multiple heads star tracker is not only of higher reliability and autonomy, but also of higher precision and dynamic performance than classical star tracker. In order to achieve the best performance, system parameters optimization for multiple heads star tracker is needed. A mathematical model of multiple heads star tracker was developed firstly, particularly on the star imaging and attitude determination. Then an analytic expression of accuracy was deduced, and the error sources affecting the attitude accuracy were analyzed. Finally, the effects of the field of view and the relative alignment on the accuracy were discussed systematically based on the simulation results. The results show that the relation between field of view and accuracy depends on the angular rate. The accuracy is improved with smaller field of view at lower angular rate, while with larger field of view at higher angular rate. And multiple heads star sensor reaches the highest precision when the boresights are orthogonal.