Currently, the digital holographic microscopy mainly uses a laser with good coherence as the light source, it needs high equipment and environmental condition, and a large amount of speckle noise and parasitic interference fringes can be introduced in the hologram. The digital holography based on incoherent light is a good way to solve those problems. The incoherent light is used as light source, and the spatial light modulator as well as the CCD composes the incoherent interference for recording holograms. The recording and reconstructed principles of the module are analyzed from the angle of wave optics and the point spread function, the axial magnification and the reconstruction distance of the system are given. Combined with the microscope objective, the system of in-line incoherent digital holographic microscopy in reflection configuration is established. Using this system to experiment on resolution imaging, a high spatial resolution as good as the traditional wide-field optical microscopy is obtained. The holograms of one diamond whose size is between 300 μm and 500 μm are captured and digital focused in different planes. The results show that the proposed system can obtain three-dimensional spatial information of small objects simply and rapidly. It has limitless applications in medical tests, material analysis and other fields.