The invention discloses a manufacturing method of a large-diameter thick-wall bimetal composite pipe. The manufacturing method of the large-diameter thick-wall bimetal composite pipe comprises the following steps of steel plate ultrasonic inspection, edge milling, pre-springing, JCO forming, straight side pressing, prewelding, outside welding, inside welding, heat treatment, rounding, weld joint X-ray examination, outer weld joint magnetic particle testing, body ultrasonic testing, hydrostatic testing, chamfering, pickling and inactivating and appearance quality inspection. The prewelding technology is plasma welding, the outside welding technology is multiple-wire submerged-arc welding, and the inside welding technology is hot wire argon arc welding. The Q245R/N08825 large-diameter thick-wall bimetal composite pipe prepared by the manufacturing method is high in low temperature impact toughness of the base layer, the mechanical properties of welded joints are quite high, and at the same time, a cladding layer in the weld joint has quite good corrosion resistance.