Disclosed herein are methods for dynamic user Identity authentication for authenticating the identity of a user of a login device. In some embodiments, the method includes the following steps: using a web server to receive an access request from the login device, generate a resource address information and a session identifier, and transmit the same to the login device; using the login device to generate an initiation signal and transmit the same to a signing device thereby initiating an air signature procedure to generate a target signature, wherein the signing device includes a motion sensor configured to sense movement features produced when the user moves the signing device; using a determination module to determine whether the target signature matches a reference signature and generate a authentication information based on the determination; and using the web server to determine whether the access request is granted based on the authentication information.