Disclosed are an S-(carboxymethyl)-cysteine pharmaceutical compound (I), and a preparation method and use thereof. Also disclosed are an S-(carboxymethyl)-D-cysteine ammonium salt monohydrate, and use thereof in preparation of medicines for preventing and treating respiratory system diseases such as chronic obstructive pulmonary diseases, in particular in preparation of expectorants. The compounds can reduce airway resistance and production of oxides in rat COPD models, increase the level of antioxidants, and alleviate damage caused by the oxides and inflammatory mediators to lungs.