The paper presents the experimental study results of the dependence of the longitudinal shielding coefficient of a magnetic shield (in the direction of the shield axis) on the strengths and mutual orientation of the operational internal and additional external magnetic fields of a small-sized rubidium atomic clock (RAC). In this case, an additional field magnetizes the RAC’s magnetic shield from the outside and penetrates inside it. The significant influence of these fields in their interrelation on the shielding properties of the magnetic shield of the RAC has been found. The obtained results allowed us to determine the longitudinal RAC-shielding coefficient at the effective values of the operational and additional magnetic fields and, as a consequence, to compensate for the effect of geomagnetic field variations on the frequency of the onboard RAC.