A fail-safe method for a parallel hybrid electric vehicle, having a motor connected between an engine and a transmission, and an engine clutch connected between the engine and the motor, includes: operating the engine using a starter and engaging the engine clutch; switching a first gearing map, which determines a change in a gear ratio of the transmission depending on a throttle vale opening rate regulated by an accelerator pedal and a vehicle speed obtained, to a second gearing map, which allows the gear ratio to change at a higher vehicle speed than that before the motor system failure occurs; and assisting a driving power of a first battery consumed by a low voltage DC-DC converter (LDC) with a counter electromotive power of the motor generated during operating of the engine in an engaged state of the engine clutch.