Abstract: Briefly described here in the paper are the chemical composition of and on-line heat treatment process for the large-sized，heavy-wall X70QS seamless linepipe.Also comparatively analyzed are the effects on the said pipe’s structures and properties by the on-line quench process and the off-line quench process separately.The analysis leads to such a conclusion that thanks to employment of the on-line quench treatment process following postrolling reheating+sizing，the said pipe possesses acicular ferrite structures both in its outer layer and core area，which brings about excellent strength，toughness and corrosion resistance.