A user identity authenticating method and device for preventing malicious harassment are described. The method includes: after a user logs in an internet website, unique first authentication code information is generated and stored; the first authentication code information and a Short Message (SMS) Uplink (UL) service number used for identifying the internet website uniquely are displayed on a website page; after the user sends an SMS including second authentication code information input by the user to the corresponding SMS UL service number, an acquired second authentication code is matched with a stored first authentication code, if the matching succeeds, then the authentication succeeds. The embodiments in the disclosure can avoid possible malicious harassment effectively, thus avoiding legal risks which a website service provider may confront to some extent, and improving the satisfaction of its service. In addition, the increase of processing load and performance influence of the website service provider's server, which are caused by triggering the sending of a large number of authentication codes maliciously, can be avoided in the disclosure.