The invention provides a preparation method for erythro form ritodrine hydrochloride shown as formula I. The method includes: subjecting a compound II and a bromination reagent to alpha bromination reaction to obtain a compound III, then conducting nucleophilic substitution with a compound IV to synthesize a compound V, performing deprotection to generate (1-(4-hydroxyphenyl)-2-[2-(4-hydroxyphenyl)ethylamino]propyl-1-one (VI), and then conducting reduction synthesis of ritodrine, performing splitting, and finally adding hydrochloric acid to form ritodrine hydrochloride. The method provided by the invention can significantly reduce the isomer impurity D, and can achieve efficient preparation of the medicinal purity product.