The tuna polypeptide (TP) was used as the reducing agent and the stabilizing agent to synthesize a tuna polypeptide selenium nanoparticle (TP-SeNP) via a green method. An animal experiment was conducted to investigate its immunomodulatory and antioxidant effects in vivo. The results indicated that the TP regulated the accumulation and stabilization of the TP-SeNP. And the conversion of selenium was tested to be 20.44%. The TP-SeNP was about 22 nm in diameter, a mix of spherical and quasi-spherical, and amorphous. The reaction between the TP and Na2SeO3 was entropy-driven spontaneous, and the binding force was mainly hydrophobic. Intake of the TP-SeNP could greatly increase the phagocytic activity of the mononuclear phagocytic system, and the contents of immunological molecules. The antioxidant capacity of the liver was also improved.