The Shanghai FEL User Facility (SFEL) for interdisciplinary studies is based on a rf linear accelerator. The prime goal of SFEL is to provide a broadly tunable laser beam from near-IR to far-IR with tens of MW at peak power. A linear accelerator will operate in three modes: ~ 3 MeV mode, 20 ~ 30 MeV mode and 40 ~ 50 MeV mode. In 20 ~ 30 MeV mode, the accelerator consists of a ns grid gun driven at 476 MHz, a 476 MHz subharmonie buncher, a 2856 MHz T-W type of buncher with high field gradients, and a SLAC type linac.