Abstract: Objective The Study of Relationship between Circular Orbit Cone Beam Reconstruction Error and Cone Angle. Method : Compared with 2-D CT reconstruction, Cone-Beam reconstruction has several advantages, and now it has been implemented more and more widely. Circular orbit based Cone-Beam reconstruction algorithms, which are mathematically simpler and more feasible than other algorithms, are more favorable. However, because of the limitation by circular orbit, we cant obtain all the data in Radon Space which means that the result becomes better with the cone angle decreases. So it is important to study the relationship between the cone angle and the reconstruct quality further. Results : In this paper we obtain a relationship between the error and the cone angle with some approximation. Conclusion : Then we propose some ways to improve circular orbit based Cone-Beam algorithms.