The invention discloses a device and method for preparing LNG from methanol-to-olefins methane tail gas. The device comprises a first-stage heat exchanger, a second-stage heat exchanger, a feed pipe,a gas-liquid separator and a rectifying tower. The methanol-to-olefins methane tail gas passes through the feed pipe, is cooled through the first-stage heat exchanger and then is fed into the gas-liquid separator for gas-liquid separation. C2 and C2+ liquid separated from the gas-liquid separator are output from the bottom of the gas-liquid separator. The gas separated from the gas-liquid separator passes through the second-stage heat exchanger to be fed into the rectifying tower for rectifying. An LNG product obtained through rectifying is output from the bottom of the rectifying tower. The gas left after rectifying is output from the top of the rectifying tower. Methane, C2 and C2+ in the methanol-to-olefins methane tail gas are extracted out through low-temperature separation, the liquefied LNG product is obtained, and the methane tail gas is effectively utilized. Consumption of raw materials is reduced, and the operation cost is lowered. Through the process, effective components inthe tail gas are recycled, the profit of the device is increased, and the device has good economic benefits.