Second harmonic generation (SHG) imaging needs no fluorescent labels and brings no photobleaching or toxicity. It's a promising tool for high-resolution, three-dimensional studies in biomedicine fields. Backward SHG imaging system is established base on a converted laser-scanning two-photon fluorescence microscope. This epifluorescence configuration is especially advantageous to thick tissue imaging and in situ detection. Various tissues were tested to find out those which give rise to strong backward SHG. Then backward SHG imaging was applied in the studies of human skins. The SHG intensity from gangrenosis skins was much lower than that from normal skins. The skins of diabetic patients, which showed no macroscopic abnormality, also displayed a distinct decrease in SHG intensity. The result indicated that collagen in these skins may have suffered from micro-structural disorder or other changes. SHG imaging may serve as a new diagnosis method for dermopathy or other diseases.