In order to improve the three FOVs position and orientation determination equipment reliability, the basic constraints test probability distribution model of the three FOVs system was established. Based on the model, the optimized FOV size of the equipment was analyzed on the visible band. First, the necessary conditions of the composite FOVs star identification and the sample FOV star identification were analyzed, and the probability distribution models of the methods were established. The reason why probability distribution of recognized triangles on three FOVs equipment cannot be simply deduced from the one FOV′s was illuminated in theory and simulated experiment. Then, the optimal value of the probability distribution was obtained by the monte Carlo method. Finally, the optimal design of the FOV size was presented by computer simulation. The experimental results show that: the three FOVs celestial navigation equipment using the composite FOVs star identification is more reliable. The optimal FOV size is 4.225°×3.168 75°. It can satisfy the reliability requirements of the three FOVs position and orientation determination equipment.