A degradation estimation method system of a high voltage battery is provided that detect degradation of a battery. The degradation estimation method includes obtaining, by a controller, information about an input voltage, an input current and an external temperature of a slow charger. The controller is configured to calculate an output power used for charging the battery, based on an input power calculated based on the input voltage and the input current. In addition, the method includes calculating, by the controller, a mean charging current based on the output power and the battery charging voltage and calculating a battery degradation degree based on the mean charging current.