Embodiments of the present disclosure provide a multi-band antenna and a terminal device. The multi-band antenna includes a feedpoint, a matching network, a capacitor assembly, a radiation portion, and a grounding portion. The feedpoint, the matching network, the capacitor assembly, the radiation portion, and the grounding portion are connected in sequence. The matching network includes at least a serially-connected inductor and a grounded capacitor or inductor. The grounding portion is electrically connected to a ground plane. A first resonant circuit is formed from the feedpoint to the grounding portion. The first resonant circuit generates a first resonance frequency and a second resonance frequency. The first resonance frequency is used in a CRLH mode, and the second resonance frequency is used in a half-wavelength loop mode.