Circuit structures of air-conditioner power supply for diesel locomotive and electric locomotive were summarized, and key points of air-conditioner power supply design were discussed. Two types of air-conditioner power supply circuit i.e. AC/DC/AC and DC/AC convert circuit were compared in detail, advantages and defects of which were pointed out. At the same time, principle of air-conditioner variable frequency speed regulation was introduced, as well as the characteristics of fix-frequency and vari-frequency air-conditioner was compared.