Abstract: Directional angle is the most basic parameter in subsea equipment navigation during offshore 2D seismic survey. So far, the magnetic declination of electronic compass should be calibrated to get the real directional angle. Declinometer can provide real time calibration parameters. In this article, the principle of magnetic declination measurement is discussed, and the reasons for closure error in residual measurement angle during navigation arithmetic process for 2-D offshore seismic survey. Finally, a brief introduction is given to the current application of declinometer.