Abstract: For conventional manufacturing process for the seamless steel tube,the TMCT that is generally used as a microstructure-trimming technique for steel plate production is not adopted,resulting in disadvantages i.e.,the alloy addition amount is obviously higher than that for the steel plate in the same steel grade,and any high-strength product can’t be obtained without subsequent heat treatment,which means much higher costs and energy consumptions. Based on the on-line TMCP technology that has been successfully applied to manufacture of the hot-rolled seamless steel tube on an industrialization basis over the recent years,the effect by the cooling process on the product as-rolled microstructure properties is studied. As a result,it is demonstrated that thanks to employment of proper TMCP path,the as-rolled microstructure can be effectively trimmed,and compared with the regular as-rolled tube that is subjected to air-cooled,the tube’s tough- ness can be obviously improved,which shows the potential for producing the hi-strength seamless steel tube. With proper chemical composition as designed,the Q620 hi-strength seamless steel tube has been produced successfully with all of its properties satisfactory to applicable specification.