In contrast to the traditional bulk optical elements, metasurfaces provide a new perspective on flexibly shaping the electromagnetic field by manipulating its phase, amplitude as well as polarization at will via a compact and easy-of-fabrication system. However, due to the high phase dispersion of their building blocks, metasurfaces significantly suffer from large chromatic aberration. Such chromatic aberration significantly degrades the performance of full-color optical applications, such as in communication, detection, imaging, displaying etc. Here we propose a design principle to realize achromatic metasurface devices which successfully eliminate the chromatic aberration over a continuous wavelength regions from 400 to 660 nm and 1200 to 1680 nm in a transmission and reflection scheme, respectively, for circularly-polarized incidences. For this proof-of-concept, we demonstrate broadband achromatic metalenses which are capable of focusing light with arbitrary wavelength at the same focal plane. A broadband achromatic gradient metasurface is also implemented, which is able to deflect wide-band light by the same angle. Through this approach, various flat achromatic devices that were previously impossible can be realized, which will allow innovation in full-color detection and imaging.