The invention provides a protective gas energy saving method and system for an online copper pipe machining annealing device. The method comprises the following steps that A, discharging is conducted; B, straightening is conducted, and a workpiece is straightened; C, heating annealing is conducted, during annealing, a nitrogen pipeline is adopted for gas supplying to conduct annealing oxidation preventing protection, a heat exchange area of a gas-water heat exchanger is calculated and determined according to the heating power of the online annealing device, the flow of the protective gas in a heating annealing area and the flow of circulating cooling water in an insulating cooling area, accordingly, an adaptive gas-water heat exchanger is selected, after the nitrogen pipeline serves as a bypass and is connected to the gas-water heat exchanger for heat exchange, water way circulation is conducted through a circulating cooling water tank, and after the gas-water heat exchanger conducts heat exchange and temperature rise, nitrogen is supplied; D, temperature reducing and cooling are conducted, water way circulation is conducted through the circulating cooling water tank, and the workpiece is cooled accordingly; and E, clamping and conveying are conducted, and the workpiece is clamped and conveyed to the next procedure. The energy saving system adopts the above method.