The invention belongs to the technical field of smelting fused magnesite by using magnesite as raw materials, and particularly relates to a process and a device for directly preheating materials by magnesium melting lump afterheat. The device is parallelly provided with two heat radiation passages and a preheating passage arranged between the two heat radiation passages, a smelted high-temperature magnesium melting lump is conveyed into the heat radiation passages to be cooled, the high-temperature magnesium melting lump is crystallized, cooled and solidified for releasing heat, after the crystallization is completed, the magnesium melting lump enters a hulling chamber for hulling and then returns back to a kiln body again, hot air in the heat radiation passages is exhausted to the preheating passage by an interlayer wall body, and the materials are preheated to 300 to 400 DEG C.