Abstract: Defatted Decapterus maruadsi powder was obtained by low-temperature continuous phase transition. The effect of low-temperature continuous phase transition on the basic components of defatted Decapterus maruadsi powder was studied. Fourier transform infrared spectroscopy(FT-IR)and circular dichroism(CD)were used to investigate the effects of this technique on the secondary structure of protein in defatted Decapterus maruadsi powder. Then alkaline protease was adopted for hydrolysis to prepare antioxidant peptides,orthogonal design was used to optimize hydrolytic condition,and the antioxidant activity of Decapterus maruadsi peptides produced by hydrolysis was measured by DPPH free radical scavenging rate. As a result,after being removed fish oil by low-temperature continuous phase transition extraction technology,the water content of the defatted Decapterus maruadsi powder was 5.38%,the ash contents was 4.29%,the crude fat was 0.23%,and the crude protein was 83.78%.The results of FT-IR showed that there was no significant change in the rigidity and toughness structure of protein in defatted Decapterus maruadsi powder. After further verification by CD,it was found that the secondary structure of protein was maintained completely in defatted Decapterus maruadsi powder. It could be concluded that the low-temperature continuous phase transition extraction technology does not destroy the structure and activity of protein in defatted Decapterus maruadsi powder. The optimal conditions of alkaline protease for hydrolysis were as follows:Ratio of water to raw material 25(mL/g),enzymatic temperature 50 ℃,enzyme dosage 8000 U/g·pro,enzymatic pH9.5. Under these conditions,the DPPH free radical clearance rate was 89.99%±0.13%,which exhibited strong antioxidant activity.