Code offloading is promising to accelerate mobile applications and save energy of mobile devices by shifting some computation to cloud. However, existing code offloading systems suffer from a long communication delay between mobile devices and cloud. To address this challenge, in this paper, we consider to deploy edge nodes in the proximity of mobile devices, and study how they benefit code offloading. We design an edge-centric code offloading system, called Echo, over a three-layer computing hierarchy consisting of mobile devices, edge and cloud. A critical problem needs to be addressed by Echo is to decide which method should be offloaded to which computing platform (edge or cloud). Different from existing offloading systems that let mobile devices individually make offloading decisions, Echo implements a centralized decision engine at the edge node. This edge-centric design can fully exploit the limited hardware resources at the edge to provide an offloading service with Quality of Service guarantee. Furthermore, we propose some novel mechanisms, e.g., lazy object transmission and differential object update, to further improve system performance. The results of a small-scale real deployment and trace-driven simulations show that Echo significantly outperforms existing