Provided are an apparatus and method for detecting a leakage current of a high-power line of an inverter of a 48V mild hybrid system. The apparatus includes a starter generator manufactured as a synchronous motor, in which a conventional field winding is wound around a rotor, and configured to generate starting torque and generating torque for an engine, a 48V battery configured to supply a 48V direct current (DC) power and be charged by the starter generator, a 12V battery configured to supply power to internal electrical components of a vehicle, an integrated starter generator (ISG) control inverter provided between the starter generator and the 48V battery and provided as a power semiconductor switch which controls a stator and the rotor of the starter generator, wherein a winding output from the inverter is connected to the rotor through a brush and a slip ring, a DC-DC converter configured to convert the power of the 48V battery into 12V battery power and supply the 12V power, a common mode (CM) choke provided between the ISG control inverter and the 48V battery, and a leakage current detector connected to the CM choke and configured to detect a leakage current.