The invention discloses a preparation method of a propylene copolymer with low energy consumption and high ethylene content. The method disclosed by the invention adopts a gas phase multi-temperature range technology, a second external electron donor is selectively added under the presence of a highly active ziegler-natta catalyst, so that propylene, or propylene and ethylene can perform propylene homopolymerization and copolymerization in at least three continuous polymerization areas of one or more polymerization reactors. The method disclosed by the invention adopts the gas phase multi-temperature range technology, at a low temperature area, most of polymerization activity of the catalyst is ensured, and at a high-temperature area, the heat exchanger efficiency of a heat exchanger is improved; in addition, the second external electron donor is selectively added, and the second external electron donor is matched with a solid catalyst component, containing titanium, of the ziegler-natta catalyst, so that the reactivity ratio of ethylene in the polyreaction can be effectively increased, the high ethylene content of products is ensured, and besides, the concentration of the ethylene in gas phase composition is reduced; therefore, the energy consumption during production is reduced, and the production capacity of the conventional device is reached.