The invention relates to a titanium carbide sheet layer/molybdenum sulfide nanosheet/titanium dioxide nanosheet composite and a preparing method thereof. The method includes the steps of conducting the mixing reaction on titanium aluminum carbon and hydrofluoric acid, conducting the hydrothermal reaction on an obtained precipitate A, adding a molybdate thiopolybdate solution to a precipitate C obtained through the hydrothermal reaction, and conducting the photo-reduction reaction to obtain the titanium carbide sheet layer/molybdenum sulfide nanosheet/titanium dioxide nanosheet composite. The composite is applied as a photocatalyst in water photocatalytic decomposition. The composite is an efficient photodecomposition water catalyst with a precious-metal-free double-aid catalyst, and the photocatalysis effect is remarkably improved.