The invention discloses a preparation method of high-purity cubic phase sodium niobate nano powder, which comprises the following steps: proportioning niobium oxalate or niobium ammonium oxalate and sodium chloride according to the mole ratio, and dissolving in deionized water; (2) grinding in an agate mortar while dropwisely adding anhydrous ethanol; (3) drying the obtained precursor in a vacuum drying oven; (4) heating the dried precursor powder in an energy-saving box furnace, keeping the temperature, and naturally cooling to room temperature; (5) carrying out centrifuge washing on the obtained powder with a deionized water-anhydrous ethanol mixed solution; and (6) drying the obtained powder in a vacuum drying oven to obtain the cubic phase sodium niobate nano powder. The method solves the problem of difficulty in preparing cubic phase sodium niobate; the preparation technique has the advantages of short time consumption and low energy consumption, and is suitable for large-scale production; and the prepared cubic phase sodium niobate nano powder has the advantages of high adsorption capacity, high response sensitivity to visible light, high photocatalytic efficiency and the like.