The invention discloses a thermal treatment method of a 15CrMoG steel annular welding line, which has better effect of preventing delayed cracks and is more stable. The thermal treatment method comprises the following steps of: wrapping the welding line of the electric heater by applying a crawler belt type electric heater and an electric heater automatic temperature controller in signal connection with the electric heater; covering a heat insulating layer which has the thickness of 45-55mm and is made from aluminum silicate cottons outside the electric heater; carrying out thermal treatment on a part of the welding line with a temperature-raising speed of 150-220 DEG C/h; controlling the temperature-raising speed to be no less than 205*25/delta and no more than 330 DEG C/h when the temperature is raised to 400 DEG C, wherein the delta is the thickness of a wall of a base metal and the unit is mm; keeping the temperature for 45-70min when the temperature is raised to 650-750 DEG C, and then instantly lowering the temperature with the temperature-lowering speed of 100-260 DEG C/h; and naturally cooling the welding part after the temperature is reduced to 400-300 DEG C. The method greatly reduces crack occurrence rates after welding, improves the welding quality and the work efficiency with stable welding quality, is mainly applied to the thermal treatment of the 15CrMoG steel annular welding line, and can be used as a reference for preventing the crack of linear welding line of 15CrMoG steel and other heat-resistant alloy steel types.