Since the United States launched the first synthetic aperture radar satellite in 1978, it has attracted widespread attention because of its ability to obtain high-resolution images of almost every corner of the earth, all day, all weather, and regardless of national borders and politics. At present, spaceborne SAR has been widely used in civil and military applications. With the complexity of spaceborne SAR application scenarios, different imaging widths and imaging resolutions are required to deal with different imaging scenarios, imaging targets, and imaging tasks. For different imaging requirements, different imaging algorithms have been proposed, including strip imaging mode, spotlight imaging mode, scan imaging mode, etc. It is of great significance to integrate multiple imaging modes in a spaceborne SAR. Different imaging modes have different requirements for antenna beam pointing. And each imaging mode has the need to quickly switch beam pointing. The phased array antenna can realize the switching of beam pointing with different beam control codes, and the speed of beam direction switching depends on the calculation speed of beam control code. Therefore, the calculation strategy and the calculation speed of the beam control code have become the main problems which restricting the multi-mode imaging of spaceborne SAR. In this paper, a real-time antenna beam control method based on the computing framework of DSP, FPGA and CPU is proposed. According to different imaging modes, the corresponding beam control code calculation strategy is designed. The test results show that relying on the high-performance computing power of DSP and the logic control capabilities of FPGA and CPU, it perfectly meets the needs of quickly completing beam control code calculation to realize beam pointing switching in different imaging modes. This method makes it possible to realize multimode imaging of spaceborne SAR.