The invention provides preparation of a WO3 (tungsten trioxide) nanoflower material and application of the WO3 nanoflower material in a gas sensor. A preparation method of the WO3 nanoflower material includes: adopting a NaOH leaching process to extract tungsten from scheelite so as to obtain tungstenic leach liquor; adding the leach liquor into an HCI (hydrogen chloride) solution to form a tungstic acid precipitate, and dissolving washed tungstic acid by adding deionized water and H2O2; adjusting pH of a mixed solution to 1.2-1.8 by the HCI solution, and reacting for 4-16 hours at a constant temperature in a range of 100-180 DEG C to obtain a WO3 nanoflower assembled from nanosheets automatically. The WO3 nanoflower is 300-420 nanometers in diameter and 100-140 nanometers in thickness. Each of the nanosheets is 170-390 nanometers in length, is 120-140 nanometers in width, is 30-50 nanometers in thickness and is provided with a hexagonal structure. The gas sensor is prepared by coating the WO3 nanoflower on a gold electrode on an outer surface of a ceramic tube and aging. By preparing a NO2 gas sensor on the basis of the preparation method of the WO3 nanoflower material, quick responses to high selectivity and low working temperature of low-concentration and even ppb-level (part per billion-level) NO2 gas can be realized.