The present invention is related to an adjustable harmonic filtering device, mainly comprising a first connecting port, a second connecting port, a harmonic filtering unit, and an adjusting unit, in which a passive network is presented between the first connecting port and the second connecting port, as well as the harmonic filtering unit is connected to the passive network. The harmonic filtering unit comprises a first inductor and a first capacitor. The adjusting unit is adjacent to the first inductor of the harmonic filtering unit, and induced electromagnetically therewith. Thus, frequency-band of the harmonic, to be filtered out by the harmonic filtering unit, may be changed, allowing for reducing loss of signal occurring in the process of transmission between the first connecting port and the second connecting port effectively.