Petroglyphs vividly record the production, life, worship and other activities of ancient ancestors. They are epics of human civilization. However, there are great difficulties in dating rock petroglyphs. In the paper, a method using digital image processing based on a Hue, Saturation and Intensity (HSI) color model is presented to date the petroglyphs. In theory, rock Intensity (I) is linearly correlated with the age of the rock and therefore can be used to date petroglyphs. This method can extract the rock intensity in the entire rock, which is much better than using a limited number of points by manual testing. The experimental results show that this method can provide a reasonable, simple and quick solution to the problem of dating rock paintings. It should be mentioned that the accuracy is dependent on the accuracy of the dating of the rock selected.