The invention relates to an application of united use of a chitosan nanometer-level acoustic contrast agent and Dickkopf-2 gene to preparation of medicines for treating prostate cancer. For the firsttime, the inventor finds relationship of Dickkopf-2 gene and prostatic cancer, the expression of the Dickkopf-2 gene in tissues of patients suffering from prostatic cancer is obviously reduced, the chitosan nanometer-level acoustic contrast agent is used as a transmission carrier, the Dickkopf-2 gene is directionally conveyed to tumor tissue, and through expression of the Dickkopf-2 gene, proliferation of tumor cells is restrained. The prepared chitosan nanometer-level acoustic contrast agent adopts chitosan as shell membrane materials, and is safe, non-toxic, good in biocompatibility and lowin cost. The particle diameter is in the nanometer-level scope, and the chitosan nanometer-level acoustic contrast agent can smoothly pass through the gap of vascular wall of tumor tissue, so as to deliver target genes to the tumor tissue, and through united use of an ultrasonic target destroy microvesicle technique, the gene transfection efficiency can be notably improved.