Abstract: According to the Huygens-Fresnel principle,Debye formula is revised.By using the revised Debye formula the diffraction of a converging spherical wave at a circular aperture is studied and the results are compared in detail with those obtained by using the Li-Wolf formula and original Debye formula.It is shown that the revised Debye formula is still valid for the low Fresnel number near the geometric focus and can give focal shift that can not be predicted in the use of the Debye formula.