Using the image processing function and numerical calculation function of MATLAB to analyze the grain size of clastic rock, the method can better solve the problems of inaccurate measurement results as well as time-consuming and laborious work in traditional grain size analysis methods. Firstly, the image of clastic rock under the microscope is processed by grayscale, binarization and image enhancement, and all kinds of granularity data in clastic rock are measured accurately. Then, the obtained data are transformed into grain size parameters and applied to sedimentary environment analysis to calculate sedimentary environment more accurately. The results show that this method not only has high measurement accuracy, simple performance, convenient operation and good applicability but also shows that the analytical results are consistent with the traditional particle size analysis method.