This paper concentrates on the signal processing of Arc FMCW SAR, a novel concept of GB-SAR mode. The signal model and its properties are analyzed firstly. Based on the analysis, a new focusing algorithm for Arc FMCW SAR is developed, which accurately take the signal properties of Arc FMCW SAR into account. Therefore, it is more practical and accurate comparing to their counterparts. Simulation experiment validates the reasonableness of the algorithm. The first Arc FMCW SAR real data experiment was carried out in IECAS and the first Arc FMCW SAR image was obtained by the proposed algorithm, showing high performance of the presented focusing algorithm in focusing as well as phase preserving.