Many low-voltage converters are adopted in direct air-cooled auxiliary power systems, and a lot of harmonics are produced on the inlet and outlet sides of converters. In this paper, harmonics of direct air-cooled auxiliary power systems were calculated based on a 2×600 MW direct air-cooled power plant; the field measured data validated the calculation results; and harmonic control measures were put forward. KEYWORDS: direct air-cooled；auxiliary power；converter； harmonic analysis；harmonic control measure