As one of the oldest painting art forms, the mural is an extremely important cultural relic. This kind of painting, which expresses the author’s ideas, decorate buildings, propagates ideas and expresses religious worship through images and colors, exists in every aspect and corner of historical life. Because of the religious activities, production and other human factors intervention in history, there are a large number of sooty murals in grottoes and temples. The contents and styles of murals cannot be identified by conventional methods. We still cannot completely remove the sooty layer on the murals with the effective technical means without any damage, so the study and appreciation of murals are seriously affected. Because of the different absorptivity of all kinds of pigments used in the murals under the infrared light, we can achieve the distinction. Using the property of infrared spectrum which can penetrate the surface of the material, the investigation and study of the sooty murals can be identified clearly. Infrared photography has been used in the study of the murals in large buddhist grottoes for a long time, but it has not been widely used in the study of ancient murals, especially in small-scale and medium-scale grottoes. In this paper, the sooty murals of the MutisiGrottoes which are the representation of small-scale grottoes in the Southeast of Gansu were studied by using infrared photography. It provides important information about the contents of the images, the ink lines of the manuscripts and even the modification marks which cannot be observed in visible light. It is also helpful to infer the date of the painting. The local details can be further obtained by adjusting the tolerance and color levels of the infrared images. This paper makes up for the lack of infrared photography in the study of small-scale and medium-scale grottoes. At the same time, it can also be used as detailed information on the present preservation status of murals, which is helpful for the protection and restoration of murals. Infrared photography has a great significance for the study of ancient sooty murals with non-contact, lossless, fast and intuitive properties. This method can be popularized in the study of the murals in the grottoes and temples and plays a key role in the study of ancient sooty murals.