There have been many studies on the evaluation of the uncertainty of the focal length of convex lensand the methods to reduce the experimental error.lt focuses on how to correctly select the appropriate focallength and aperture diameter of convex lens to reduce the experimental error,puts forward the important factorcausing the experimental error-the relationship between the focal depth and the lens parameters , and studies theinfluence of paraxial light on the experimental error.The research results provide a new way to reduce theexperimental error,and further study the law of lens imaging.