The invention discloses a method for preparing natural fiber with flame retardance. The method comprises soaking, peeling, scrubbing and dividing procedures carried out in sequence; after dividing, kudzu vine peel fiber is soaked in a flame-retardant finishing liquid, taken out after soaking, squeezed for liquid removal, and dried in an oven after liquid removal. The flame-retardant finishing liquid comprises components: boric acid, a phosphate fire retardant, polyamine, a CWS cationic softening agent, sodium hypophosphite, magnesium chloride and water. After the kudzu vine peel fiber is processed by the flame-retardant finishing liquid, the flame retardance of the kudzu vine peel fiber is in remarkably improved. A fabric made of the kudzu vine peel fiber processed by the flame-retardant finishing liquid is detected, and a result shows that the limit oxygen index can reach 28-30 and the water washing durability is excellent.