NOVELTY - Extracting protein from residual sludge comprises: (a) mixing fresh residual sludge and deionized water to prepare sludge liquid; (b) adding into papain, stirring and hydrolyzing, inactivating enzyme in boiled water, centrifuging, and discarding the precipitate to obtain the protein supernatant; (c) using an isoelectric point precipitating method to obtain the protein precipitate, centrifuging to obtain the sludge crude protein; and (d) drying the crude protein precipitate at 50 degrees C for 48 hours and cooling at room temperature to obtain the product.    USE - The process is useful for extracting protein from residual sludge (claimed).    ADVANTAGE - The process is simple, low in cost, low in reaction temperature, short in extracting time, small in damage to protein, high in extracting rate of protein in sludge, and appropriate for industrial production. Moreover, the environmental pollution is reduced; and the reduction and the reclamation of the sludge are achieved.    DETAILED DESCRIPTION - Extracting protein from residual sludge comprises: (a) mixing fresh residual sludge which is digested and dehydrated and deionized water according to solid-liquid ratio of 1:4-6 to prepare sludge liquid; (b) adding into papain with concentration of 5-6%, stirring and hydrolyzing for 3-6 hours at constant temperature of 50-60 degrees C, inactivating enzyme in boiled water for 10-20 minutes, centrifuging for 10-20 minutes at speed of 3600 rotations/minute, and discarding the precipitate to obtain the protein supernatant; (c) using an isoelectric point precipitating method to obtain the protein precipitate in the sludge extracted solution, centrifuging at pH value of 4.5-6.5 and at 4 degrees C with speed of 10000 rotations/minute to obtain the sludge crude protein; and (d) drying the crude protein precipitate at 50 degrees C for 48 hours and cooling at room temperature to obtain the product.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow diagram of the process for extracting protein from residual sludge (Drawing includes non-English language text).