The utility model relates to a high-temperature pressure reducing diffusion furnace which comprises a quartz tube. One end of the quartz tube is a quartz tube opening. A quartz door is arranged at the quartz tube opening. The other end of the quartz tube is connected with a process gas exhaust pipe. The quartz tube opening comprises a quartz tube inner end opening and a quartz tube outer end opening which share the same circle center. The quartz tube outer end opening is placed on the outer side of the quartz tube inner end opening. The radius of the quartz tube outer end opening is larger than the radius of the quartz tube inner end opening. The quartz door is arranged at the quartz tube inner end opening. The quartz tube outer end opening is provided with a stainless steel furnace door. The quartz door and the stainless steel furnace door form a transition cavity. A buffer fixing device is arranged between the quartz door and the stainless steel furnace door in the transition cavity. The stainless steel door is provided with an air blowing air inlet through which air is led into the transition cavity and an air blowing air outlet through which air is exhausted from the transition cavity. According to the high-temperature pressure reducing diffusion furnace, the transition sealed cavity is arranged, pressure difference at two ends of the quartz door and stress on the quartz door are greatly reduced, design of the quartz door is simplified, and stress damage of quartz is greatly lowered.