Abstract: The precipitated phase of TP347HFG steel under high temperature stress was studied by transmission electron microscopy and scanning electron microscopy. The results show that the precipitated phases were M₂₃C₆ and NbC carbides with face-centered cubic structure. The former was distributed in the grain and grain boundary, and the latter was dispersed in the grain. With the extension of test time, the number of M₂₃C₆ increased significantly, but the size did not change significantly. There were many primary carbides in TP347HFG steel. Under the action of long-term high temperature stress, carbides were easy to accumulate at grain boundaries and their vicinity, which promoted the formation of cracks.