Abstract: Investigated in the article is the relationship between the micro-structural distribution status of the 12Cr1MoVG heavy-wall seam less steel tube and its surface hardness.Based on the measured CCT curve of it，the optimum heat treatment process for the Φ 219mm×45mm 12Cr1MoVG heavy-wall tube is determined，involving such critical parameters as furnace quenching：temperature 970 ℃，time 60minutes；water quenching：time 20 s；tempering：temperature 750℃，time 150minutes.The actual operation result of the said heat treatment process demonstrates that with proper control of the quenching time，the ferrite segregation amount can be reduced，so as to prevent remarkable segregation of the martensitic structure，and thus get satisfactory control of the product hardness.The newly-set heat treatment process ensures that the hardness range of the 12Cr1MoVG heavy-wall tube is consistently kept as 161～182 HB，and the surface hardness qualification rate thereof is up to 100%，while all the other mechanical property data are in compliance with relevant requirements under the applicable product standard.