The invention discloses a method for extracting water-soluble collagen. The method comprises the following steps: firstly slicing a tissue rich in type III collagen; then cleaning tissue slices and soaking in 0.1-40%(W/V) alkali solution for 0.5-24 hours; cleaning until pH is 5-8, then soaking for about 0.5-48 hours with normal saline, draining excessive moisture, mashing the tissue at the temperature of 2-8 DEG C, then dispersing into enzymatic hydrolysate, and carrying out enzymatic hydrolysis for 1-72 hours in an environment at the temperature of 2-40 DEG C; after the enzymatic hydrolysis is finished, regulating pH value to be 7-13, and standing for 2-24 hours at the temperature of 2-40 DEG C; filtering with a filter screen of 30-150 meshes, and removing substances which are not subjected to enzymatic hydrolysis; salting out with 20-50%(W/V) salt solution, collecting solid contents, carrying out dialysis with a dialysis bag with the molecular weight cut-off of 4000-8000, and removing excessive ions and impurities, so that polymorphic collagen is obtained; and finally, dispersing a polymorphic collagen mixture into water, regulating pH value to 1-5, filtering with a filter screen of 30-150 meshes, removing precipitates, salting out filtrate, and then carrying out dialysis, so that the target collagen is obtained. By adopting the method for extracting the water-soluble collagen, the obtained collagen is an aqueous solution, and the blank that existing collagen is acid soluble or alkali soluble is filled up.