The effects of different aging processes on the precipitated phase, mechanical properties, molten salt corrosion resistance and post-weld microstructure of 347H stainless steel were studied. The results show that a large number of precipitated phases appear in the crystal after aging at 700 °C for 400 h. After aging for 3000 h, the number of precipitated phases increases and most of them are gathered at the grain boundaries. There are two forms of precipitates, one is the coarse precipitate rich in Cr, and the other is the smaller precipitates mainly consisting of NbC. After aging at 700 °C for 30 min, the yield strength and tensile strength of the samples at room temperature and 593 °C increase, but the elongation decreases. The corrosion results in nitrate at 565 °C show that the corrosion products of the aged samples are the same as that of the original samples, which are Fe₂O₃, Fe₃O₄, MgCr₂O₄, MgFe₂O₄, FeCr₂O₄ and NaFeO₂. The proportion of Fe₃O₄ that is dense and well bonded to the subtrate in the original sample is higher than that in the aged sample, so the corrosion resistance is better. At 700 °C, the aging time has no obvious effect on the microstructure after welding.