The surface flaws and internal knots in a film of high power laser lens are experimentally measured by using a near field micro-image method, and their formation mechanisms are analyzed as well. The evanescent waves radiated by a conic tip at a 100 nm diameter interact with the defects embedded in the films. After the evanescent waves are converted into radiation waves, they are collected by the objective lens and imaged point by point in the far field. Atomic force microscopy (AFM) images and scanning near-field optical microscopy (SNOM) images on the surface of the thin film are obtained synchronously, so as to visually identify the physical mechanisms of the defects formation. The results show that the surface flaw and the internal knot in thin film are accurately identified at the same time in effective interacting areas of evanescent wave. By comparing AFM result with SNOM result, we find that the surface flaw of substrate accumulates the residual stress in the processing of coating, which results in a layered cracking on the surface of film. The crosswise profile scale of single minimum flaw is 165 nm, which is beyond experimental detection precision of traditional far field detection. In addition, the high hot spot in SNOM graph shows that refractive index of knots exist in the thin film is higher than that in substrate.