The invention provides a preparation method of a levomilnacipran diastereoisomer. The preparation method comprises the following steps: taking benzyl cyanide and R-epichlorohydrin or S-epichlorohydrinas starting materials, performing cycloalkylation on the starting materials in an alkaline condition, performing separation to obtain (1S, 2S)-2-hydroxymethyl-1-phenyl cyclopropylcarboxylic acid or (1R, 2R)-2-hydroxymethyl-1-phenyl cyclopropylcarboxylic acid, performing a reaction on the (1S, 2S)-2-hydroxymethyl-1-phenyl cyclopropylcarboxylic acid or (1R, 2R)-2-hydroxymethyl-1-phenyl cyclopropylcarboxylic acid and acetic anhydride to obtain ethyl ester, performing a chlorination reaction, performing a reaction on a reaction product and dimethylamine to form dimethylformamide, performing hydrolysis to obtain (1S, 2S)-2-hydroxymethyl-N, N-diethyl-1-phenyl cyclopropanecarboxamide or (1R, 2R)-2-hydroxymethyl-N, N-diethyl-1-phenyl cyclopropanecarboxamide, performing a reaction on the (1S, 2S)-2-hydroxymethyl-N, N-diethyl-1-phenyl cyclopropanecarboxamide or (1R, 2R)-2-hydroxymethyl-N, N-diethyl-1-phenyl cyclopropanecarboxamide and phthalimide after chlorination and removing phthaloylwithhydrazine hydrate, so as to obtain the levomilnacipran diastereoisomer. The method is simple, variables are controlled easily, and the levomilnacipran diastereoisomer can be prepared efficiently. Additionally, the prepared levomilnacipran diastereoisomer is relatively high in purity.