Abstract: The demand for oil casing steel with ultra-high strength and excellent impact toughness for safe application in ultra-deep wells is pressing. In improving the combination of strength, ductility, and impact toughness, the designed Cr–Mo–V micro-alloyed oil casing steel was quenched at 800, 900, and 1000°C, followed by tempering at 600, 680, and 760°C, respectively, to obtain distinct microstructures. The results showed that the microstructure of the samples quenched at 800°C followed by tempering comprised untransformed ferrite and large undissolved carbides, which considerably deteriorated tensile strength and impact toughness. For other conditions, the nucleated carbides and the boundaries are key factors that balance the tensile strength from 1226 to 971 MPa and the impact toughness from 65 to 236 J. From the perspective of carbide, optimal precipitation strengthening is achieved with a smaller carbide size obtained by a low tempering temperature of 600°C, while larger-sized carbides would remarkably soften the matrix to improve the toughness but deteriorate the tensile strength. Additionally, an increase in prior austenite grain size with the corresponding enlarged sub-boundaries obtained by high quenching temperatures substantially diminishes grain refinement strengthening, dislocation strengthening, and the energy absorbed in the crack propagation process, which is unfavorable to strength and toughness.