In optical 4f system with coherent illumination source,the output images are easily contaminated by dusts and spots in the surface of lens and charge coupled device (CCD). With the image degradation factors being classified by additive stochastic noise,spots,illumination ununiformity and system low-pass characterization,a simplified model is proposed. The output result is used as apriori information when the input is a full white image to calculate the distribution of spots and illumination ununiformity of which stabilization is assumed in an experiment. An improved adapted bilateral filter is therefore introduced to denoise and remove spots. The optical and synthetic image experiments show that the proposed method can effectively reduce noise and restore image with edge sharpness being preserved. The robust characterization of the proposed method is embodied by the simulation results even under the serious spot-contamination circumstance.