Optical vortices, carrying quantized orbital angular momentum (OAM) states, have been widely investigated because of their promising applications in both classical and quantum realms. Among these applications, efficient generation and measurement of OAM beams are critical. Current techniques available for generating OAM beams generally suffer from bulky size, low operation efficiency, or single-function or complicated fabrication processes. Here we propose and experimentally demonstrate an approach to generate arbitrary optical vortices with a chip-scale device based on all-dielectric two-dimensional (2D) polarization-independent metasurface grating. Based on multi-beam interference in principle and nanofabrication techniques for implementation, our device allows efficient and simultaneous on-chip generation of multi-channel beams with different OAM. We further demonstrate that our device can also work reversely in detecting the OAM spectrum of various pure and mixed optical vortices (e.g., fractional OAM orders) with low crosstalk. Our scheme may find potential applications in developing new integrated photonics for OAM-based high-dimensional quantum information processing in future quantum network.