Abstract: In order to develop hot work die steel which can be used for service at 700-900℃, the microstructure and properties of new austenitic hot work die steel are studied in this paper. The results show that the microstructure of the high-Mn as-cast electroslag remelting (ESR) ingot is composed of γ-Fe, V(C,N), Mo₂C. V(C,N) is irregular multilateral strip or slice shape with severe angles. Most of eutectic carbides Mo₂C are lamellar fish-skeleton-like, except rod-shaped. With increasing solid solution time and temperature, the Rockwell hardness increases first and then decreases, and the austenite grains increase slowly. When the solution temperature exceeds 1 170℃, the austenite grain size increases linearly. The optimal combination of hardness and austenitic grain size can be obtained by soaking for 2 hours at 1 170℃. The maximum Rockwell hardness is 47.24 HRC, and the corresponding austenite average grain size is 58.4 μm. Compared with the single-stage aging, the maximum impact energy of the specimen after two-stage aging heat treatment reached at 16.2 J and increased by 29.6%, while the hardness decreased by 1 to 2 HRC. The optimum aging heat treatment process is subjected to pre-aging treatment for 1 hour at 650 ℃, and finally by re-aging for 1 hour at 780 ℃.