Abstract: This paper presents an integrated L-band radiometer–scatterometer system based on a shared receiver for ground calibration and validation of spaceborne active–passive microwave payloads. To address the high cost, hardware complexity, and limited flexibility of conventional ground-based multi-instrument systems, a receiver-sharing scheme is proposed, in which the antenna, RF front-end, and digital baseband resources are shared, and a single receiver chain is multiplexed in time for passive brightness temperature acquisition and active backscatter reception. Benefiting from a configurable RF and digital baseband platform, an automated dual-mode measurement strategy is developed to unify data acquisition, processing, and output within one system. Experimental results demonstrate that the system can stably obtain both backscattering coefficients and brightness temperatures, and that their angular responses are generally consistent with scattering and radiometric models. The proposed system provides a feasible solution for compact ground-based active–passive microwave observations, and can support the ground calibration and validation of spaceborne L-band active–passive microwave payloads, as well as related geophysical parameter observation and ionospheric propagation effect analysis.