The invention provides a medical collagen membrane with a compact outer layer and a loose inner layer in structure. Fish skin pepsin-solubilized collagen is dissolved in an acid solution to prepare collagen solutions with different concentrations, then laying is performed according to the concentrations from high to low, and pressing is performed to prepare a collagen membrane, wherein the fish skin pepsin-solubilized collagen is prepared by treating fish skin with alkali liquor, and then carrying out enzymolysis with pepsin under a weak acid condition; an enzymatic hydrolysate is centrifuged, a supernate is collected, and the supernate is salted out; precipitates are collected, the precipitates are redissolved with a weak acid solution, and then dialysis treatment is performed; and a dialysate is freeze-dried to prepare the fish skin pepsin-solubilized collagen. The collagen membrane prepared in the invention has three layers of different structures, the aperture of the inner layer is relatively large to promote the growth of osteoblasts, the small aperture of the outer layer can prevent soft tissue cells from entering and allow stem cells and nutrient substances to pass through, and a certain transition layer exists between the inner layer structure and the outer layer structure to facilitate tissue integration.