Based on the vectorial Debye theory, the tightly focusing properties of Bessel-Gaussian radially polarized beam through a high numerical aperture (NA) with a diffractive optical elements (DOE) and a dielectric interface are investigated. A three-dimensional optical chain along optical axis can be obtained by designing the DOE to spatially modulate the incoming Bessel-Gaussian radially polarized beam. Research results show that the focusing shape in the focal area can be controlled by changing the interception ratio of the aperture radius to the incident beam waist radius, numerical aperture and the refractive index of the medium in the focal region. When the interception ratio is beyond to a constant value, a focal dark channel can be obtained. The length of the focal dark channel increases with the increasing of refractive index of the medium in the focal region in case of not changing the width of the focal dark channel.