The invention discloses a preheating welding apparatus for a marine equipment pile leg, comprising a welding system and a base platform; the welding system includes a tubular electromagnetic heating device sleeving the pile leg, the electromagnetic heating device is peripherally provided with an induction coil, the tubular wall of the electromagnetic heating device is provided with an open groove with the outer wall provided with an automatic welding device; the base platform is provided with a driving device for driving the electromagnetic heating device to rotate relative to the pile leg and a guide support structure for supporting the electromagnetic heating device, and the base platform may slide relative to the pile leg. The pile leg is preheated by means of electromagnetic induction heating, and the apparatus may heat pile legs with different shapes and diameters, is highly universal and high in heating efficiency, and no interior and exterior temperature difference problem occurs to the pile leg. The electromagnetic heating device and the automatic welding device are rotated, with better simplicity and convenience and improved welding efficiency and welding quality provided. The invention also discloses a preheating welding method for a marine equipment pile leg.