NOVELTY - Preparing selenomethionine comprises: (a) dissolving substituted amino acid compounds (a1) into an organic solvent and adding a reductant to obtain alpha -amine protected homoserine compounds (a2); (b) cyclizing (a2) to obtain homoserine lactone compounds (b1); (c) dissolving (b1) into the organic solvent to react with selenium compounds (c1) to obtain diselenohomocystine compounds (c2); (d) reacting (c2) with the reductant and reacting with a methylation reagent to obtain alpha -amine protected selenomethionine compounds (d1); and (e) deprotecting (d1).    USE - The process is useful for preparing selenomethionine, which is useful as an intermediate in drugs.    ADVANTAGE - The process avoids the use of toxic and offensive dimethyldiselenide, thus the process greatly reduces the damage to the environment and to operators, and hence the process is environmentally friendly. The process produces selenomethionine in high productivity, low production cost and high purity.    DETAILED DESCRIPTION - Preparing selenomethionine comprises: (a) dissolving substituted amino acid compounds of formula (R1aO2C-CH2-CH(NHR)-C(=O)-OH) (a1) into an organic solvent and adding a reductant to generate alpha -amine protected homoserine compound of formula (HO-CH2-CH2-CH(NHR)-C(=O)-OH) (a2); (b) cyclizing (a2) under the catalytic function of an organic acid, inorganic acid or Lewis acid to obtain homoserine lactone compounds of formula (b1); (c) dissolving (b1) into the organic solvent to react with selenium compounds of formula (M2Se2) (c1), adding water to mix with the reacted product, extracting the mixed solution by an ether or ethyl acetate, and depressurizing and concentrating the obtained organic layer to obtain diselenohomocystine compounds of formula (RNH-CH(CO2H)-CH2-CH2-Se-Se-CH2-CH2-CH(CO2H)-NHR) (c2); (d) reacting (c2) with the reductant and reacting with a methylation reagent to obtain alpha -amine protected selenomethionine compounds of formula (CH3-Se-CH2-CH2-CH(CO2H)-NHR) (d1); and (e) deprotecting (d1), where the selenomethionine and the intermediates are in L or D configuration or in a racemic mixture.    M = Li, Na or K;    R = alkyloxycarbonyl group of formula (R2-O-C(=O)-), substituted carboxybenzyl group of formula (1a), acetyl, substituted benzenesulfonyl group of formula (2a) or substituted triphenyl group of formula (3a);    R2 = methyl, ethyl, tert-butyl, isopropyl, isobutyl, trichloromethyl, cyclohexyl, tert-pentyl, diphenylmethyl, adamantly, allyl or 4-pyridyl methyl;    R1a = methyl, ethyl, isopropyl, benzyl, p-nitrobenzyl, acetophenone group or allyl; and    R3-R12 = H, methoxyl, nitro, hydroxyl, F, Cl, Br or I.