Abstract: In recent years, the demand for high-strength and high-toughness petroleum casing pipes has been growing continuously in the drilling and exploitation of deep and ultra-deep wells. In this study, a high-strength and high-toughness petroleum casing steel was employed as the research material. The influences of soaking temperature and soaking time during heat treatment on the original austenite grain growth behavior were systematically investigated. The effect of original austenite grain size on the strength and toughness of the experimental steel was characterized using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that within the temperature range of 780-820 ℃, the original austenite grain size increases with the increase of soaking temperature and the prolongation of soaking time. The original austenite grain size has a significant influence on the microstructure after quenching and tempering. A finer original austenite microstructure eventually transforms into finer martensitic laths, accompanied by a higher proportion of high-angle grain boundaries in the microstructure, thereby contributing to better strength and toughness. A kinetic model of austenite grain growth for the experimental steel was obtained through linear regression analysis of the experimental data. The results and conclusions of this study can provide a theoretical foundation for the formulation and parameter optimization of the heat treatment process for high-toughness petroleum casing steel.