The invention provides a method for circularly extracting protein and humus from residual sludge after dehydration by combining an ultrasonic method with an acid process and relates to a method for extracting the protein and humus from the residual sludge. The method disclosed by the invention can be used for solving the technical problems that the existing sludge recycling method only can extract single substance from the sludge, maximum use of resource can not be realized and biodegradability is poor. The method disclosed by the invention comprises the following steps: firstly, ultrasonically cracking the dehydrated residual sludge which is produced by a municipal sewage treatment plant under the acidic condition; secondly, standing and settling for promoting precipitation of protein; thirdly, centrifuging and extracting the protein; fourthly, filtering and separating the humus, and recycling filtrate. The method has the advantages that the protein and humus can be extracted at the same time, extraction is complete, biodegradability of the sludge left after extraction is increased to 75.7% from 40.2%, and the method can be applied to the field of resource recycling treatment of municipal sludge.