The invention discloses a furnace door device of a diffusion furnace. The furnace door device of the diffusion furnace comprises a furnace door executing structure, a furnace door supporting assembly, a furnace door, an inner quartz door and an adjusting assembly, wherein the inner quartz door is connected with the furnace door. The adjusting assembly comprises an adjusting pillar and a plurality of extension arms, and the extension arms are arranged in the radial direction of the adjusting pillar. The adjusting pillar is installed in the center of the furnace door, the included angles between the two adjacent extension arms are same, one end of each extension arm is fixedly connected with the adjusting pillar, and an adjusting component is installed at the other end of each extension arm. The adjusting components abut against the furnace door, one end of the furnace door supporting assembly is connected with the furnace door executing structure, and the other end of the furnace door supporting assembly is connected with the adjusting pillar. The furnace door device of the diffusion furnace has the advantages that the structure is simple, adjusting is convenient, and airtightness is good.