The invention relates to the field of coal chemical wastewater treatment, in particular to a device and a method for purifying methanol-to-olefin device washing water. The method comprises the following steps: pumping out the washing water from the bottom of a washing tower, and sending to a granular-bed separator; enabling the washing water to run through the granular bed in a horizontal direction, intercepting particulate matters in water, and agglomerating microfine oil droplets in the water under the oil substance adsorbing and agglomerating actions of medium particles in the granular bed;performing settling separation on the agglomerated oil droplets under the action of gravity; after the granular bed adsorbs the particulate matters in the water till a saturated state, feeding wastewater from the bottom of the granular bed, mixing with gas, fluidizing the medium particles till a boiling state, releasing solid pollutants between bed layers, and sending the pollutants to a solid concentrating unit along with backwashing liquid. By the method, separation of catalyst particles and the oil substances from the washing water is achieved.