A method of forcibly regenerating a gasoline particulate filter (GPF) includes a first operation of starting an engine, a second operation of determining whether a coolant temperature in the on-state of the engine is equal to or lower than a predetermined temperature, a third operation of determining whether an idle state of the engine is maintained for a predetermined time or longer when the coolant temperature is equal to or lower than the predetermined temperature, and a fourth operation of switching a fuel injection to an air-fuel (A/F) lean injection and forcibly regenerating the GPF by performing the A/F lean injection when the idle state of the engine is maintained for the predetermined time or longer.