For a 3-D distributed space-borne SAR, small area antenna leads to serious Doppler ambiguity. Joint processing of all echoes from multiple antennas is able to eliminate Doppler ambiguity and obtain high resolution and wide swath. This paper firstly builds the echo model for the formation of one transmitting and two receivings. Then the relationship between the two received echoes is analyzed. After compensation for envelope difference and part of phase difference, the analytic expression which describes the frequency spectrum relationship between two echoes can be obtained accordingly. Based on this equation, two weight functions are designed to eliminate the Doppler ambiguity. Finally, a conventional imaging algorithm is used to get SAR images.