The invention discloses a preparation method of a stannic oxide nanocrystalline loaded tin disulfide nanosheet composite nanomaterial. The method comprises the following steps that S1, a tin disulfide nanosheet is prepared, wherein L-cysteine and stannic chloride are dissolved in water and are mixed evenly, light yellow sediment is obtained after a hydrothermal reaction, rinsing, centrifugal separation and drying are carried out, and the SnS2 nanosheet is obtained; S2, a composite material is prepared, wherein stannic chloride and vitamin C are dissolved in water, the SnS2 nanosheet prepared in the S1 is dispersed in the solution, then NaHCO3 is added, even mixing is carried out, a microwave reaction is carried out under the backflow condition, cooling, rinsing, centrifugal separation and drying are carried out, and the composite nanomaterial SnO2/SnS2 is obtained. The molar ratio of the L-cysteine to the stannic chloride added in the S1 is 6-8:1, and the molar ratio of the stannic chloride added in the S2 to the SnS2 nanosheet is 1-5:1. According to the prepared SnO2/SnS2 composite material, SnO2 nanocrystalline is evenly dispersed and firmly loaded on the SnS2 nanosheet, the caking phenomenon of the SnO2 nanocrystalline is avoided, and the obviously-enhanced catalytic property is shown.