The invention discloses a slag treatment system and a slag treatment method of a biomass boiler. The system comprises a slag groove, a slag conveyer and a reburning device. The reburning device is hermetically arranged between the slag groove and the slag conveyer and can move along a horizontal direction. The reburning device is controlled to be closed, the slag falls onto the reburning device through the slag groove so that the reburning of carbon in the slag is realized. Heat generated by burning of carbon in the slag returns to a hearth. The reburning device is controlled to be opened, and the slag falls onto the slag conveyer and is discharged. The method comprises the steps of: closing the reburning device so that the slag falls onto the reburning device through vibration of a fire grate, the fire grate stops vibrating, and heat generated by reburning of carbon in the slag returns to the hearth; opening the reburning device so that the slag falls onto the slag conveyer and is discharged; and repeating the steps to realize the reburning of carbon in the slag and recycling of the heat. According to the slag treatment system and the slag treatment method of the biomass boiler provided by the invention, heat of the slag is recovered and the boiler efficiency is improved through the reburning of unburned carbon in the slag by the reburning device.