The SmCo alloys are widely used in the field of precision instruments and high temperatures due to their excellent temperature stability. At present, the effect of high temperature on the domain structure of SmCo alloy lacks sufficient study. In this paper, the effect of temperature on the magnetic domain of type 2:17 SmCo alloy was investigated by domain structure observation and micromagnetic simulation. After heating at 240 ℃ and 300 ℃, the reverse domains could be observed on the surface of the magnet, and the domains nucleated at the center region of the grains and then expanded into maze domains. The in-situ heating test using Lorentz electron microscope confirms, from a microscopic point of view that thermal activation causes irreversible movement of the walls of the domains. It was found by the comparison between batch heating tests and continuous heating tests that part of the inverted domains are unstable during the cooling from high temperature down to room temperature, and the micromagnetic simulation results verified that without the temperature field and magnetic field, the unstable inverted domains would disappear under the influence of the interaction, and the domains pinned by the phase boundary will be stably retained. The micromagnetic simulation results reveal that the nucleation sites of the reverse domains in the SmCo alloy are at the intersection of the 2:17 R phase and Zr-rich phase, which is due to the lower pinning field of the Zr-rich phase.