A horizontal pyrolysis furnace has a kiln and two barrels. The two barrels are respectively a processing barrel rotatably disposed in the kiln and a takeover barrel detachably connected with the processing barrel. Each one of the two barrels has a gate assembly and at least one spiral guiding plate. The gate assembly of the processing barrel is mounted on an end of the processing barrel, and extends out from the kiln. The two gate assemblies of the two barrels are detachably connected such that the two barrels are able to rotate synchronously. The at least one spiral guiding plate is fixed on an inner surface of one of the two barrels, and the spiral guiding plates of both barrels have an identical helical direction.