Based on the scalar diffraction theory, a binary annular phase plate is studied by means of the coherent superposition method, which is used for the generation of a hollow focal field. The analytical expression of the radius for each ring is derived. The influence law of the ring radius on the hollow focal field is also investigated by the numerical calculation method. The research results show that, in the focal plane, the ring radius of this phase plate has a great influence on the side lobe ratio and the maximum light intensity ratio, but has a little influence on the radius of the hollow focal field. The increase in the number of rings reduces the side lobe radio, but increases the maximum light intensity ratio. In contrast, in the axial direction, the increase in the number of rings cannot increase the maximum light intensity ratio. Both the number of rings and the ring radius have relatively great influence on the side lobe ratio and the maximum light intensity ratio. For a binary annular phase structure, the side lobe ratio and the maximum light intensity ratio only depend on the ring radius and are independent of numerical aperture. In the focal plane, the radius of the hollow focal field is inversely proportional to the numerical aperture. In the axial direction, the radius of the hollow focal field is inversely proportional to the square of numerical aperture.