The invention provides a closed automatic slag discharge device for a boiling chlorination furnace. The slag discharge device comprises a slag discharge tube, a slag tank and a slag storage funnel, wherein the slag discharge tube is slantways arranged on a boiling section of a furnace body downward; the input port of the slag discharge tube is communicated with an inner cavity of the furnace bodyand the output port of the slag discharge tube is communicated with the input port of the slag tank; the output port of the slag tank is formed in a cavity body of the slag storage funnel; and in theslag discharge device, a first material seal is formed through furnace slag retained in the slag tank and a second material seal is formed through furnace slag discharged to the slag storage funnel. The closed automatic slag discharge device can realize closed automatic overflow and continuous slag discharge based on an automatic overflow theory under the condition that the height of a bed layer in the boiling chlorination furnace is in direct proportion to the material quantity and the differential pressure of the bed layer; during slag discharge, the chlorine supply and the material supply are not required to be stopped; in the whole production process, the height of the bed layer of the boiling chlorination furnace is kept stable automatically, so stable and high fluidization quality can be obtained and the chlorine utilization rate can be increased remarkably; moreover, two material seals are formed through the furnace slag in the slag tank and the furnace slag in the slag storagefunnel and the chlorine cannot be leaked, so the working environment and the working condition are improved basically.