The invention provides a large-diameter thick walled pipe postweld heat treatment method. By adopting the large-diameter thick walled pipe postweld heat treatment method provided by the invention, insulating temperature is determined according to a hardness range of a base material in the postweld heat treatment process, simultaneous heating of an inner wall and an outer wall is adopted, a heatingwidth is determined according to a formula, a power of a heater is determined according to a formula, and thus the heat treatment quality of a weld crater is effectively ensured, a temperature difference between the inner wall and the outer wall of the weld crater in postweld heat treatment is lowered to meet the standard requirement or a higher standard to avoid the phenomenon of base material overburning or difference between the weld crater properties of the inner wall and the outer wall caused by a temperature deviation, weld crater heat treatment can be ensured to mold at a time at the same time to avoid the phenomenon of heat treatment again, the heat treatment effect is improved, and the economic benefit is very considerable.