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 fullcolor 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.