Abstract: In the high-resolution, wide-swath imaging application of spaceborne SAR, the technique of Digital Beam-Forming (DBF) can be employed to receive and process echo data with multiple channels on elevation, increase the receive gain and Signal-to-Noise Ratio(SNR). According to the geometric relationship among these channels, this paper analyzes the impulse response of the elevation channel, and derives the explicit expression of it. Based on this, this paper proposes a DBF processing scheme which combines the time-variant weighting and Finite Impulse Response (FIR) filtering, and further presents the block diagram of system realization. Compared with other DBF methods, this approach can maximize the receive gain and optimize the system performance for the long pulse, without increasing the complexity of the digital beam-former hardware. Simulation results indicate that it allows the system to achieve the theoretically optimal performance.