Firstly, Methionine (I) is alkylated and then brominated to form bromohomoserine bromate (III). Secondly, Selenium reacts with three reducing agents to obtain disodium diselenide, and then reacts with compound III to obtain selenocysteine (IV); Finally, Selenomethionine (V) was formed by reduction of sodium triacetoxyborohydride and methyl iodide. The main steps were optimized and the optimal synthesis conditions were obtained: the preparation of disodium diselenide with a new reductant sodium triacetoxyborohydride could obtain a higher yield of intermediate product IV, and the highest yield of compound III was 62.42% when the molar ratio of sodium hydroxide to sodium triacetoxyborohydride was 4.0:1.0. When the molar ratio of compound IV to iodomethane was 1.0:2.0 and the molar ratio of sodium hydroxide to sodium triacetoxyborohydride was 2.7:1.0, the highest yield of final product V was 75.79%; The structure of the final product V was confirmed by mass spectrometry, hydrogen spectrum and carbon spectrum after crystallization and purification (purity 95%).