A fast plasma boundary reconstruction technique based on a local expansion method is designed for EAST. It represents the poloidal flux distribution in the vacuum region by a limited number of expansions. The plasma boundary reconstructed by the local expansion method is consistent with EFIT/RT-EFIT results for an arbitrary plasma configuration. On a Linux server with Intel (R) Xeon (TM) CPU 3.2 GHz, the method completes one plasma boundary reconstruction in about 150 µs. This technique is sufficiently reliable and fast for real-time shape control.