To enhance the accuracy of Positioning,Navigation,and Timing (PNT) services in underwater environments,a no- vel optical high-precision bidirectional time comparison method is proposed.This paper elucidates the basic principle and system components of this method and describes the construction of an eXperimental system in a glass pool utilizing a laser diode,beam collimator,and mirror.The time comparison results eXhibit a peak-to- peak value of 1.6 ns,with an additional time transfer jit- ter of 1.05e — 11 s/s and 6.30e — 11 s/103 s.The eXperimental results verify the feasibility of the optical bidirectional time compari- son method in underwater space,and the nanosecond time comparison results show that it is valuable for further research.Mo- reover,a detailed analysis of eXperimental challenges provides a foundational understanding for future work and investigations in this area.