The invention provides a hemp fiber with a fineness of 1,500-4,000 Nm and the strength of 4-10cN / dtex. The hemp fiber has higher fineness and strength and can be used for spinning by using a dry method and preparing various grades of textile products by blending with other fibers. The invention also provides a preparation method of the hemp fiber, comprising the following steps of: chemical degumming, aftertreatment, stamping, oil feeding, softening, oil feeding and humidification, stowing and mechanical opening, wherein the step of chemical degumming specifically comprises the following steps of: adding hydrogen peroxide, alkali metal silicates, urea and a penetrating agent to water and uniformly stirring; then adding hemp peels; heating up, preserving heat and washing by using clear water; adding alkali, the alkali metal silicates, urea and the penetrating agent to water and uniformly stirring; adding oxidized hemps; and heating up, preserving heat and washing by using clear water. The method can prepare hemp fibers with an extremely high fineness and has low environmental pollution.