The invention discloses a preparation method and sensing application of a gold nanorod thin film. The gold nanorod thin film is prepared by completely and uniformly mixing a gold nanorod aqueous solution and an agarose aqueous solution and then performing vacuum drying. The obtained gold nanorod thin film has very high uniformity; furthermore, the density of the gold rod can reach nearly a hundred times of the density of the gold nanorod solution due to the shrinkage effect of hydrogel in the drying process. Meanwhile, in the presence of agarose linear macromolecules in the film, the god rods have high single dispersibility, so that the prepared gold nanorod film has a high optical characteristic. The prepared gold nanorod film is applied to construction of a visual immunosensor, and the sensor shows rich color change in the presence of target objects with different concentration, so that the target objects can be subjected to visual semiquantitative analysis through visual inspection.