Treprostinil is a synthetic prostacyclin derivative with thrombocyte aggregation inhibitory and vasodilatory activity. Treprostinil can be administered in subcutaneous, intravenous, inhalable, or oral forms. Disclosed is a method for the preparation of treprostinil of formula I and its amorphous form, anhydrate form, monohydrate form, and polyhydrate form salts with bases. In the disclosed method, the chiral center in the 3-hydroxyoctyl substituent is formed at the end of the synthesis, so that the method is robust and well scalable. Also disclosed are treprostinil intermediates and the preparation of the intermediates.