At present, multi-resolution analysis has become a popular method in pixel-level image fusion. However, those algorithms based on multi-resolution analysis with decomposition and reconstruction transform process is too complex and not suitable for real-time image sequence fusion. In order to meet the requirements of real-time fusion system, a novel Fast Mutual Modulation Fusion (FMMF) algorithm for multi-sensor images is proposed. First, the two source images were magnified by factors that were derived from the ratio of the corresponding pixel energy respectively. Then, an offset entry obtained by computing statistical parameters of source images is added to it. Finally, after the previous results are multiplied and normalized, the fused image is obtained. Fusion process consists of the addition and multiplication, which is a nonlinear combination process. Experimental results show that FMMF algorithm is simple and fast and its performance and efficiency are superior to those based on pyramid and wavelet.