Abstract: We propose the design of anisotropic metamaterials with cascading meta-atoms. Each meta-atom array behaves as an impedance-tuned interface and dramatically modifies the complex reflection and transmission coefficients. By engineering the frequency-dependent impedances, the reflection phase difference along the two axes of anisotropic metamaterials approximates to a constant in a wide range. We numerically demonstrate the proposed anisotropic metamaterials can accomplish achromatic polarization transformation from 0.5 THz to 3.1 THz. The polarization conversion ratio is higher than 80%, which exhibits excellent agreements with the theoretical calculation. Such design is scalable to other bands and can provide helpful guidance in broadband devices design.