A method, device for identifying service flows and a method, system for protecting against a denial of service attack are provided. The method for identifying service flows includes: detecting a user access to a target system; dynamically generating a set of user identifier information according to the detected user access to the target system and a preset user access statistical model; when the service flow needs to be identified, extracting the user identifier information from the service flow; comparing the extracted user identifier information with the user identifier information in the set of user identifier information to determine whether they are matched; determining whether the service flow is legal service flow according to the comparison result.