High dynamic range imaging (HDRI) has become a topic of intense research interest in many fields. One kind of HDRI system based on the spatial light modulator (SLM) has a problem with output image instability because of feedback modulation. The HDRI system is introduced, and then a feedback modulation model for the HDRI system is established to discuss the oscillation reason for the output images. The relationship between the input light intensity and the feedback progress is analyzed. A method that adjusts the modulate index of the SLM is proposed to solve the instability problem, and the stability conditions are given to obtain stable images. Lastly, experimental images prove that the feedback model is correct and the output image with high dynamic range is stable.