Abstract Based on quasi-conformal mapping approach, we propose a two-dimensional broadband and low-loss planar lens in the acoustic regime, which can be realized using gradient-index sub-wavelength artificial structures with hard boundary. In this design, QCM is employed in the acoustic regime to convert a parabolic form into a planar one, which makes the resultant material approximately isotropic through minimizing the anisotropic factor. It is found that the planar lens can show the same performance as its parabolic counterparts over a broadband range of frequency. Moreover, the lens is fabricated via 3D printing technology and measured experimentally, which gives identical results with the theoretical prediction. The proposed technique could be extended to realize different acoustic devices and applied for a series of practical applications.