The invention discloses a fused salt heat diffusion treatment device and application thereof. The fused salt heat diffusion treatment device comprises a gas supply system, a vacuum system, a furnace body, a circulation water coil, a sample chamber, a crucible and a supporting rod. The gas supply system communicates with the furnace body through a gas inlet pipe. The vacuum system communicates with the furnace body through a first gas outlet pipe. The upper portion of the furnace body is sleeved with the circulation water coil. The crucible is arranged at the bottom of the furnace body and comprises a crucible body and a crucible cover plate. A first annular groove is formed in the outer side of the upper end of the crucible body, and an opening of the crucible cover plate downwards wraps an opening of the crucible and stretches in the first annular groove. The supporting rod sequentially penetrates a furnace door on the furnace body and the crucible cover plate and stretches in the crucible. The end, stretching in the crucible, of the supporting rod is connected with the sample chamber. The fused salt heat diffusion treatment device is high in treatment efficiency, the system is airtight, the water oxygen content of the system can be controlled, fused salt steam or decomposed products of the fused salt steam are prevented from being diffused outside the crucible so that influences of fused salt fog can be reduced, and the phenomenon that the device is corroded or the fused salt fog is accumulated on a resistance wire of a heating furnace to cause short circuits is avoided.