A terahertz ultra-narrow band absorber based on metal ring structure is proposed. The unit cell of the absorber adopts typical metal-dielectric-metal structure, the top metal pattern consists of closed metal ring and four-split metal ring, and the bottom layer is continuous metal plate. The narrow band absorption principle of the absorber and the surface current distribution of the absorber structure at the absorption peak frequency were studied. The research shows that the absorber possesses a narrow absorption peak at 1.7682THz, the absorption is 99.8%, the full width at half maximum relative to the resonance frequency is 0.51%, and it is polarization insensitive to the polarization of x and y incident waves. The absorber characterized by simple structure and easy processing has potential application value in bio-sensing, narrow band thermal radiation and photoelectric detection.