A vehicle power supply circuit monitoring system and a vehicle power supply circuit monitoring method are provided. The vehicle power supply circuit monitoring system is adapted to a vehicle power supply and a vehicle load device, and includes a power monitoring circuit and a control circuit. The power monitoring circuit is electrically connected to the vehicle power supply and the control circuit. The power monitoring circuit monitors two different input load powers at two different time points. The control circuit calculates an external power loop equivalent impedance according to the two different input load powers. When the external power loop equivalent impedance is greater than a designed impedance threshold, the control circuit executes a warning procedure.