Impedances of different portions of active implantable medical devices (AIMDs) can influence its magnetic resonance imaging (MRI) radio-frequency (RF) safety dramatically. To better understand this behavior, in this article, we analytically derive relationships between the impedances of AIMDs and the MRI RF safety. Numerical simulations and measurements are performed to validate the proposed relationships. In addition, effective measurement setups for the AIMD impedances are proposed. The analysis in this article gives a clear design strategy for MR-conditional safe AIMDs.