Abstract: As the use of high-strength thick plates is increasing in marine engineering, bridge engineering, petroleum pipelines, and other fields, the required performance level of thick welded plates is also increasing. Oxide metallurgy technology, which is used to improve the toughness of heat-affected zones by controlling the formation and dispersion of high-melting-temperature oxide particles in steel, has attracted increasing attention by researchers in recent years. The effect of cerium on the welding performance of industrial quenched and tempered high-strength steel was investigated. Using a Gleeble 3500 thermal simulator, the coarse-grained heat-affected zones of high-strength steel were simulated with different cerium contents. The microstructures, austenite grains, and mechanical properties of the heat-affected zone were investigated by using optical microscopy, scanning electron microscopy equipped with energy dispersive spectrometry, and hardness testing. The results show that when the heat inputs are 25 kJ·cm⁻¹ and 50 kJ·cm⁻¹, the impact energies of the heat-affected zone of Ce-undoped steel are 84.8 J and 24.5 J, respectively. When the mass fraction of Ce is 0.0018%, the impact energies of the heat-affected zone are 110.0 J and 112.0 J, respectively. The different degrees of toughness of the two experimental steels indicate that the appropriate content of rare earth element can effectively improve welding performance. By comparing and analyzing the microstructures and prior-austenite grain sizes of the two experimental steels, it can be seen that with increases in the welding heat input, the microstructure of the heat-affected zone of the high-strength steel gradually transforms from martensite and lower bainite to upper bainite and granular bainite, and the average size of the prior-austenite grains in the heat-affected zone obviously increases. However, at the same welding heat input, the size of the prior-austenite grains in the heat-affected zone of Ce-doped high-strength steel is significantly smaller. The observed microstructure of Ce-doped steel is finer with a reduced content of brittle upper bainite, which significantly improves the welding performance of 700 MPa high-strength steel.