Abstract: The Gleeble 3800-GTC thermal simulation testing machine was used to simulate the welding thermal cycling test under zero load conditions, and the microstructure evolution law and nickel element partial aggregation phenomenon of 800 MPa grade 10Ni5CrMoV high-strength steel during the thermal cycling process were studied through the analysis of the microstructure and micro-area composition of the sample. The results show that there is a nickel segregation zone in the whole thickness of the test rolled steel plate, and the nickel element diffuses downhill during the thermal cycle. During the warming period (1 000~1 460 °C), the base metal structure changed from the original linear strip nickel enrichment zone to a curved nickel segregation zone, continued to heat up to a curved nickel segregation zone and a reticular nickel segregation region along the austenite grain boundary, and continued to heat up and evolved into a wider reticular nickel segregation region along the austenite grain boundary and a spherical nickel segregation area remaining in the original segregation zone. During the cooling period (1 460~1 000 °C), the aggregation of the reticulated partial concentration area remained generally unchanged, and the remaining spherical segregation area gradually disappeared with the decrease of temperature, the grain homogenization phenomenon occurred. In the whole thermal cycle, the heating peak temperature above 1 430 °C for a long time will promote the aggregation of nickel elements at the austenite grain boundary, resulting in local grain boundary melting, cooling test temperature below 1 410 °C for a long time, and no external stress will promote the disappearance of the remaining spherical nickel segregation area of the base metal, so that the nickel content in the grain boundary will be homogenized.