Abstract Some unsteady-state solutions of Bloch equation which well-describe the behavior of a magnetometer are obtained. These solutions are in accord with the experimental result of alkali-metal magnetometer and co-magnetometer gyroscope. Many interesting phenomena can be also achieved via the solutions. First, the measuring direction of a magnetometer changes with the variation of external magnetic field along z axis. And it could be used for measuring high frequency magnetic field. Then it can be extended that the co-magnetometer without compensated field can get better performance than simple atomic magnetometer due to the effect of polarized noble gas. Finally, we discussed the limits (bandwidth and so on) of atomic magnetometer and co-magnetometer with the Bloch equation of spins. These phenomena, which have not been mentioned before, may contribute to the development of atomic magnetometer and co-magnetometer gyroscope.