By use of the Monte Carlo method, the variations of diffusive reflection with time are obtained in a scattering medium inclusive of the inhomogeneity. The variations are different when the relative position of the inhomogeneity and the light source is defferent. With combination of the difference and time-gate methods, the image of the sphere embedded in the depth of the scattering medium are acquired. The results not only show that the contrast of the image is directly related to the start-of-gate time and time-gate width, but also show that according to the brightness and darkness of diffusive reflection in the sphere projection area, the point of 80% or 50% of the maximum diffusive reflection intensity should be judged as the boundary position of the sphere.