Abstract: This paper studied the inf luence of the direct ional error of vecto r coil on the declination of FHG vector proton precession magnetometer in theory.The result indicates that only the di rectional erro r of the vector coil in east-w est direction has the inf luence on the declination. This paper also analyzed the erro r of observation data w ith the theoretical result, and f inds that there is long-period drif t about the erro r of observation data of declination and horizontal intensity of this magnetometers with the drift of directional error of the vector coil.The est imated drif t cycle ranges 2 ~ 4 months, the drift quanti ty of vertical intensity is about 3.5 ~ 11.0 nT, the declination is 0.1′~ 1.0′.The sug gestion is also proposed that the drif t must be avoided in analy sis of seismomagnetic precursor.

Key words: Vector proton precession magnetometer, Automatic, Declination, Directionalerror, Drift