An external control device includes a human-computer interface, a first controller and a power transmitting unit. An implantable medical device includes a power receiving unit, a second controller and a second detector. A method for monitoring power supply comprises: producing a first magnetic field by the power transmitting unit; sensing the first magnetic field to produce a second magnetic field and converting it into a direct current by the power receiving unit; detecting a power value of the direct current by the second detector to output a detection signal to the second controller; outputting a status information to the external control device by the second controller according to the detection signal; and receiving the status information by the first controller. The first controller transmits an adjustment signal to the power transmitting unit if informed of the status information that the power value is not within a designate power range.