Slope deformation monitoring is an important means in the prevention and early warming for landslide disaster.. The close-range photogrammetry based on image analysis has been widely used in the field of slope deformation monitoring with the advantages of full-field and non-contact. However, considering the fact that the change of environmental light reduces the gray consistency of captured slope images in the long-term field monitoring application, the close-range photogrammetry will be seriously limited in measurement accuracy and even fail. In this investigation based on the physical process of environmental lighting, the characteristics of lighting changes and their impact on image grayscale were deeply analyzed, and an image matching method based on gray gradient direction characteristics was proposed with the ability to resist the interference of environmental light. The results of indoor slope shrinkage model experiment and real field slope observation experiment show that this method can effectively suppress the influence of environmental light, improve the accuracy and stability of deformation measurement. A new idea is provided for slope deformation monitoring using photogrammetry and plays a supporting role in the practicality of photogrammetry.