An innovative design of magnetic coupler for shaft speed amplification is proposed and verified by experiments. The structure of proposed magnetic coupler is similar to an infinite-stage gearbox. In addition, the mathematical model of flux density is derived to look into the equation of adjustable gear ratio and effect of speed amplification. Moreover, two sets of experiments, namely verification of gear ratio and observation of stall phenomenon, are built up to examine the capability and drawback of the proposed variable-gear-ratio magnetic coupler. Three types of gear ratio are presented by theoretically analysis at first and then examined by experiments. The gear ratios for these three specific types between the input and output rotors are 4.75, 5.75, and 10.5, respectively. That is, the rotational speed of the output rotor can be precisely and realistically amplified. Besides, in order to reduce the torque inertia of outer rotor, a ferrite bush is inserted to the inner side of the core rotor to decrease the flux density at air gap. On the other hand, the overlapped area of permanent magnets, which are attached onto the inner rotor and outer rotor, has to be appropriately chosen. The smaller the overlapped area, the weaker is the magnetic attractive force at air gap. As long as these two modifications (an inserted ferrite bush and the aforesaid overlapped area) are validated, the torque inertia of outer rotor can be significantly reduced. Accordingly, the required power to rotate the outer rotor can be greatly reduced if the overlapped depth is shortened. However, insufficient overlapped depth between the high-speed rotor and low-speed rotor will bring about stall phenomenon caused by the magnetic attractive force between the high-speed rotor and the low-speed rotor being weaker than the start-up torque inertia. In other words, the reduced overlapped depth can also reduce the start-up torque inertia but stall phenomenon may easily occur relatively.