The invention relates to a medium and low temperature smoke online pre-ash-removal device of a rotary hearth furnace. The medium and low temperature smoke online pre-ash-removal device of the rotary hearth furnace comprises a smoke main pipe, a settling chamber, a first valve, a second valve, a bypass pipe and a third valve, wherein the first end of the smoke main pipe is connected with an air outlet of an external preheating furnace; the second end of the smoke main pipe is connected with an air inlet of an external heat exchanger; the position, close to the first end, of the smoke main pipe is provided with the first valve; the position, close to the second end, of the smoke main pipe is provided with the second valve; the position, between the first valve and the second valve, of the smoke main pipe is connected with the settling chamber; a first opening is formed in the position, between the first end and the first valve, of the smoke main pipe; a second opening is formed in the position, between the second end and the second valve, of the smoke main pipe; the two ends of the bypass pipe are in closed connection with the first opening and the second opening respectively; the position, close to the first valve, of the bypass pipe is provided with the third valve.