Abstract: The digital subsystem of the first spaceborne L-band one-dimensional synthetic aperture radiometer of China adopts a distributed structure, consisting of multiple independent, parallel working distributed front-end data acquisition units, integrated digital units, and synchronization units. The key performance of the subsystem includes phase consistency and amplitude consistency of all intermediate frequency AD acquisition channels of the distributed front-end data acquisition units. The subsystem performance tests aim at independent hardware performance and the whole digital subsystem performance. The testing of distributed front-end data acquisition units for hardware performance utilizes the synchronization pulse within the digital subsystem as a trigger signal to obtain the raw acquisition sequences of all AD channels simultaneously. The performance of the whole digital subsystem is measured by extracting the cross-correlation and auto-correlation information from the scientific data packets for processing. The ground prototype testing obtained the phase consistency of multi-channels under 1°, amplitude consistency under 0.4dB, and correlation offset under -35dB, which reaches to the instrument index requirements and also proves the correctness of the testing method.