The invention relates to a leveling agent used in ramie fabric liquid ammonia solvent dyeing and a preparation method and application method thereof and belongs to the technical field of textile chemistry. N-hydroxymethyl acrylamide is dissolved in a proper amount of deionized water to prepare an N-hydroxymethyl acrylamide solution; acrylic polyether with the polymerization degree being 45 is dissolved in a proper amount of deionized water to prepare an acrylic polyether solution; under the stirring condition, the temperature is controlled to be 79-81 DEG C, the N-hydroxymethyl acrylamide solution, the acrylic polyether solution and an ammonium persulfate initiator solution are simultaneously and slowly added into a four-mouth flask containing a certain amount of deionized water dropwise, dropwise adding time is controlled to be 2-3 h, and the temperature is preserved for 3-4 h after dropwise addition; finally, cooling is conducted to prepare the leveling agent. The weight-average molecular weight of the leveling agent is 10000-20000. The leveling agent prepared through the method is used in ramie fabric liquid ammonia solvent dyeing, the color of dyed fabric is uniform, and the dyeing defect is avoided.