The invention relates to a method and a device for preparing nitrogen in low energy consumption. The method comprises that: the nitrogen is obtained on the top of a rectifying tower after the air is cooled, and one part of the nitrogen is condensed into liquid nitrogen, wherein one part of the liquid nitrogen is taken as a product liquid nitrogen and the rest liquid nitrogen is taken as a backflow liquid to return the top of the tower; the other part of the nitrogen enters a heat exchanger, is heated again to normal temperature and pressurized by a pressure-increasing end of an expander, and is sent into a user nitrogen conveying pipe network; the liquid air rich in oxygen is obtained on the bottom of the tower, enters the heat exchanger, is supercooled by cold fluid and throttled, and enters a condenser/evaporator as a cold source to carry out heat exchange with the nitrogen; the vaporized air rich in oxygen enters the heat exchanger, is heated again, and enters the expander for expansion refrigeration; and the low-pressure air rich in oxygen after the expansion enters the heat exchanger, and is heated again and led out. The method increases the pressure of the nitrogen through work outputted by the expansion of the air rich in oxygen, and reduces pressure discharge of a compressor so as to reduce energy consumption; the liquid air is subjected to heat exchanging and supercooling by various cold fluid such as the low-pressure air rich in oxygen, the nitrogen, the pressurized air rich in oxygen, and the like, so the humidity content of the air is reduced to improve extraction yield, the energy consumption is reduced by 15 to 23 percent, the water consumption is reduced by 14 to 22 percent, and the economic benefit is significant.