Large-caliber extra-thick-walled seamless steel pipe and manufacturing method thereof

The invention provides a large-caliber extra-thick-walled seamless steel pipe and a manufacturing method thereof. The manufacturing method comprises the following steps: rolling a pipe billet into a seamless steel pipe, and performing thermal refining, wherein the pipe billet comprises the following chemical components in percentage by weight: less than or equal to 0.20 percent of C, 0.10-0.55 percent of Si, 1.10-1.70 percent of Mn, less than or equal to 0.020 percent of P, less than or equal to 0.015 percent of S, less than or equal to 0.05 percent of Nb, less than or equal to 0.10 percent of V, 0.007-0.05 percent of Ti, 0.10-0.60 percent of Cr, 0.20-1.40 percent of Ni, less than or equal to 0.35 percent of Cu, 0.08-0.60 percent of Mo, 0.015-0.050 percent of Al, 0.0005-0.0030 percent of B and the balance of iron and inevitable impurities; the thermal refining step comprises quenching and tempering treatments on the rolled seamless steel pipe; the quenching discharging pipe temperature is 900-940 DEG C; the c