The invention provides an auxiliary power supply method and device for an important system of an offshore small reactor. The method comprises the following steps: starting signals of an auxiliary power supply, an emergency power supply and a wave energy power generation device are triggered at the same time when a voltage loss signal of a first bus is triggered; when it is monitored that the auxiliary power supply and the emergency power supply cannot be started in a preset first time period and in a preset second time period respectively, a load signal of the wave energy power generation device is triggered, wherein the starting moment of the preset second time period is counted from the ending moment of the preset first time period; a first circuit breaker arranged between the wave energy power generation device and a power electronic conversion device is closed to form a power supply circuit between the wave energy power generation device and a second bus; and the wave energy powergeneration device transmits electric power adjusted by the power electronic conversion device to the second bus through the power supply circuit. According to the invention, when an offshore small reactor is powered off, power is supplied by the wave energy power generation device. The problems of common-mode failure of an existing backup power supply and large occupied space of a diesel generatorare solved.