To perform the ground test of a spaceborne Synthetic Aperture Radar(SAR) system, an echo simulator with the characteristics of multimode, multi-assemblage, and real time is proposed in this paper. A hardware platform of the real-time echo simulator based on the VPX bus is designed. With this platform, real-time multi-point echo generation and scenario targets echo generation can be achieved by the real-time signal processing in Field Programmable Gate Array(FPGA), utilizing the parameters calculated by the industry computers. Furthermore, this platform can output different signals if it is expanded to multi-channels, making it possible for the assignment of echo generation and test in different spaceborne SAR modes. The test results with the actual SAR system show that this platform can satisfy the system requirements and is now used in practice. To perform the ground test of a spaceborne Synthetic Aperture Radar (SAR) system, an echo simulator with the characteristics of multimode, multi-assemblage, and real time is proposed in this paper. A hardware platform of the real-time echo simulator based on the VPX bus is designed. With this platform, real-time multi-point echo generation and scenario targets echo generation can be achieved by the real-time signal processing in Field Programmable Gate Array (FPGA), utilizing the parameters calculated by the industry computers . This platform can output different signals if it is expanded to multi-channels, making it possible for the assignment of echo generation and test in different spaceborne SAR modes. The test results with the actual SAR system show that this platform can satisfy the system requirements and is now used in practice.