Abstract: A type of Q125 grade high-strength low-alloy oil well casing steel was designed and the effects of heat treatment processes on its microstructure and properties were investigated. Compared with the process of quenching at 870℃ and tempering at 500℃, the steel quenched at 850℃ and tempered at 500℃ has preferable combination of strength and toughness. In comparison with quenching at 870℃, the steel quenched at 850℃ has better mechanical properties because of smaller original austenite grains, which lead to a smaller size of packets and blocks. Quenching temperature has little influence on the precipitation behavior. TiN with a larger size or TiC and TiN associated precipitations can restrain original austenite grain coarsening through pitting austenite grain boundaries; TiC with a smaller size, which is rich Mo, can be an effective barrier to dislocation motion, and therefore has great contributions to improve the strength of the steel.