The invention discloses a liquid nitrogen preparation device capable of increasing yield under a single-tower nitrogen-making half-load working condition. The liquid nitrogen preparation device comprises a filter, an air turbine compressor, an air pre-cooling unit, a molecular sieve adsorber which is alternatively used, an electric heater, a main heat exchanger, a rectifying tower, a condensing evaporator, a sub-cooler, a turbo expander, a liquid increasing expander and a cold box. The invention further discloses a liquid nitrogen preparation method capable of increasing the yield under the single-tower nitrogen-making half-load working condition. According to the practical needs of a user, when dose of product nitrogen gas is reduced to be one half the normal amount or is reduced, air is introduced into the liquid expanding machine to expand, so that the yield of liquid nitrogen is increased; and compared with the conventional method, the preparation method can increase the liquid nitrogen yield by 20%-35%.