The invention discloses a synthetic method and application of polyacrylic acid-packaged nano-tricobalt tetraoxide. The synthetic method comprises the following steps: dissolving cobalt nitrate into water, so as to form a 1M-10M solution; dissolving polyacrylic acid into water, so as to form a 0.05M-0.5M solution; mixing the two solutions, adding the mixed solution into ammonia water at a continuous stirring state so as to react; continuously stirring a mixture for 24 hours so as to change the color from wine red to deep brown, and centrifuging a product at a speed of 400rpm for 30 minutes, so as to remove big agglomerates and fragments; purifying supernate, and concentrating by virtue of a 30K ultrafiltration centrifugation tube; and finally, carrying out freeze drying on the obtained solution, so as to obtain light brown solid powder, namely polyacrylic acid-packaged nano-tricobalt tetraoxide. Compared with the prior art, the synthetic method has the advantages that polyacrylic acid-packaged nano-tricobalt tetraoxide is synthesized by virtue of a one-step method, the step is simple, the cost is relatively low, and particles size of the synthesized product is relatively small; and polyacrylic acid-packaged nano-tricobalt tetraoxide is applied to the degradation of organic dyes due to high catalytic activity.