Long-term stability is a key specification for rubidium atomic frequency standards (RAFS), which is influenced by magnetic frequency shift caused by C-field stability. Taking advantage of the linear relationship between the transition frequency of ⁸⁷Rb ground state and external magnetic field, we measured the C-field intensity inside the physical package of RAFS, as well as the magnetic shielding factor of the magnetic shield case. Experimental results indicated that C-field fluctuation inside the physical package could be attenuated by using the magnetic shield case. The axial magnetic shield factor was measured to be about 3 828, which could shield geomagnetic field efficiently and keep the magnetic frequency shift in a proper range.