In this study, we first derive the light field distribution for the spatial interference with quasi-coherent or broadband light illumination, so as to obtain a four-step phase shift corrected method for such an illumination. Second, we construct a spatial interference microscope system that uses green light and diluted 6-μm polystyrene microsphere into the microscopic objective oil to simulate microenvironment of a cell. Finally, using four of the spatial-interference images, we reconstruct the phase distribution of the oil-immersed microspheres. The relative error in the microsphere radius is 6.5%, and the relative error of optical path difference volume of the oil-immersed microsphere is 8.4%. By comparing these results with those achieved by the traditional four-step phase shift method, the improvement can be neglected. Therefore, in situations where the imaging speed is important and the observed cell is sufficiently thin, it is not necessary to use the corrected four-step phase-shift method to increase the imaging speed.