Abstract: Due to the different imaging mechanisms, the multi-source images are essentially diffe- rent, which makes the fusion process different. Based on a large number of Chinese and foreign literature, this paper classifies fusion methods, and focuses on the fusion processes and typical algorithms of various fusion methods, and elaborates its key techniques in detail. At the same time, an in-depth review of the current evaluation indicators and classifications is carried out. Finally, combining the influencing factors of key techniques and the technology development status, the future development trend of the fusion image field is prospected from the five aspects such as data characteristics, time efficiency, information extraction, evaluation angles and universal applicability of the method. 

Key words: image fusion, multi-source image, fusion method, evaluation indicator