This paper is aimed at analyzing the focal field of some axially-symmetric diffractive lenses with small F-number and electrically large radii which are used in millimeter wave focal imaging, especially for the case of a plane-wave incident onto the lenses obliquely, by using the method of moments (MoM). The analysis of a diffractive lens whose radius is as large as 78.3λ can be performed using appropriate pairs of basis functions and test functions in the MoM, and specific integration methods are adopted to integrate the oscillatory integrals which occurr in the MoM when it is applied on a body of revolution (BOR) for which the rotational symmetry of the BOR has been utilized. Comparison of the simulated results with the experimental data shows that the simulation is valid. An imaging experiment is carried out to show the ability of diffractive lens in a millimeter imaging system.