Spatial resolution is a key specific parameter of the optical microscopic imaging system. According to the optical diffraction theory, the spatial resolution of imaging system is determined by the wavelength of illumination light and the numerical aperture of microscope objective. In the practical imaging process, the resolutions of microscopic imaging system obtained from different criteria are slightly different. It is necessary to select an appropriate criterion according to the coherence of light sources and the structural characteristics of obsversed targets to accurately calculate the resolutions of imaging system. In this paper, the calculation methods for spatial resolution under different conditions are provided via theoretical analysis and numerical simulation. Furthermore, we compare and discusse the difference of imaging resolutions under the illuminations of coherent and incoherent light sources for double points and double slits targets, respectively.