Abstract: A systematically study is made on heat treatment process for the casing of Grade 125 as made of the newlydeveloped A1Q steel. Compared with the conventionally-used 26CrMo4 steel,the A1Q steel features low alloy content and low hardenability,and accordingly for steel pipes with wall thickness over 12 mm,the quenching cooling rate needs to be greater than 20 ℃/s to obtain full martensite structure. It is revealed via the analysis of the technological process capability that during the early period,the process capability index is just at the qualification or insufficient level. The microstructural characterization shows that it is the mixed structure as formed during the quenching process that leads to the decrease of both strength and low-temperature impact toughness of the steel pipe. Accordingly the research focuses on the process improvement,involving rising the water temperature and strengthening the cooling rate to get the product qualification rate significantly increased,and keeping consistent. The research achievement provides important reference for the heat treatment process of the low alloy,high grade casing.