For the description of the focusing properties of a Bessel beam generated by an axicon, we first deduce a type of analytic expression other than Huygens-Fresnel diffraction integral theory. Based on the interferential theory, the analytic expression can be used to calculate the focusing beam evolution behind the focal plane of the positive lens and under the circumstance of the positive lens placed beyond the maximum non-diffraction distance, and can better illustrate the formation of the optical bottle beam. These results can not be described based on the Huygens-Fresnel diffraction integral theory. In addition, we get a different result from the one (that if z0max, hollow beam will be generated) obtained by Wei adopting geometrical optics. All of our theoretical calculations are confirmed by the experimental results.