The mirror image method can be used to solve the electrostatic field in the system with high symmetry. This paper discusses a model in which there is a circular projection on the plane of an infinite grounded conductor， and the point charge is located on the axis of symmetry of the system and has no contact with the surface of the conductor. Through the discussion on the relations among the radius of the circle，the distance between the center of the circle and the plane，and the shortest distance between the charge and the dome，three convergent models are summarized. In the first two cases，no matter where the point charge is，a stable solution can always be obtained， and the spatial electric field distribution is given by combining with MATLAB simulation. The latter case cannot be solved by the mirror image method，which reflects the limitations of the mirror image method.