A power control device configured for diagnosing an open-circuit fault occurring in a power system of an autonomous vehicle and an open-circuit diagnosis method thereof, may include a power control switch that selectively connects or separates main power output from the first power supply and auxiliary power output from the second power supply, and a processor that determines a possibility of an open-circuit fault of a vehicle power source based on a current flowing through the power control switch and determines whether it is possible to drive an electric load with an output power of the second power supply alone and determine an open-circuit position based on an output of the first power supply when there is the possibility of the open-circuit fault.