In order to better explain the basic principle of coherent anti-Stokes Raman scattering (CARS) microscopy and facilitate image processing of CARS micrograph, the method of angular spectrum is proposed to study the optical field distributions of CARS microscopy. Based on this method, lateral and axial optical field distributions as a function of the numerical aperture of objective and the spot size of incident beam are calculated, respectively. Calculations indicate that the full-width at half-maximum (FWHM) of the optical field decreases as the numerical aperture increases, and the bigger the waist radius of incident beam is, the higher the resolution of CARS imaging is. Theoretical calculations lay a solid foundation for the future experiments.