In this study, the mutual interference between two phase-locked loops (PLLs) was numerically analyzed by focusing on the electromagnetic coupling between the inductors. It was shown that the unstable state of the PLLs due to mutual interference, revealed in our previous theoretical work, can be reproduced by circuit simulation, and it was confirmed that the unstable coupling strength agrees well with the theoretical value. In addition, it was also clarified, through simulations using an electromagnetic field simulator, that the mutual coupling of inductors causes two PLLs at a certain distance to become unstable. Furthermore, to reduce mutual interference, a novel electromagnetic coupling mutual injection circuit was devised. A test circuit implementing the proposed circuit was designed using a 0.18 $\mu \mathrm{m}$ standard CMOS process.