As underwater acoustic channel is an open environment, underwater wireless sensor network (USWN) is subjected to other attacks and threats. To solve the security problem of UWSN identity authentication, we propose a UEIAP (underwater ECC identity authentication protocol). UEIAP implemented three-factor bidirectional identity authentication, which made anonymous identity dynamic and negotiates temporary session keys for secure communication in the future. Strict BAN logic formal proof and AVISPA simulation experiment show that the protocol meets the security and functional requirements with relatively little overhead and is suitable for UWSN.