Abstract: To address material failure of key components in space nuclear reactors under high-temperature and ultra-high vacuum conditions and enhance their ability to withstand severe accidents, it is necessary to develop anti-volatilization coatings with excellent thermal stability and to investigate the microstructure and high-temperature performance of the coatings. In this study, a Ni transition layer was first deposited on a nickel-based alloy substrate via direct current (DC) magnetron sputtering, followed by the preparation of a tungsten (W) coating through chemical vapor deposition (CVD) on the Ni transition layer. Volatilization experiments were conducted at 800 ℃ and an ultra-high vacuum of 10⁻⁷ Pa for 3 000 hours to investigate the effect of a Ni transition layer on the phase composition, microstructure, and thermal stability of the anti-volatilization W coatings under high-temperature conditions. The results reveal that the presence of the Ni transition layer caused the preferred orientation of the W coating to shift from (200) to (110) and significantly suppressed the “abnormal growth” of surface W grains as well as elemental diffusion at the interface. After 3 000 hours of volatilization, the surface morphology and elemental composition showed almost no changes, demonstrating excellent thermal stability and anti-volatilization performance. High-temperature led to the formation of pores at grain boundaries and elemental diffusion at the interface, resulting in partial metallurgical bonding in the interfacial region. The diffusion depth is found to be related to grain orientation, and the presence of the transition layer is observed to inhibit elemental diffusion. In summary, the DC magnetron-sputtered Ni transition layer effectively optimizes the microstructure of the W coating and significantly enhances its thermal stability under extreme high-temperature operational conditions.