Abstract: This paper presents a four-channel active-standby solid-state power amplifier (SSPA) for space-borne phased array antennas. The proposed architecture adopts a "one-active, three-cold-standby" redundancy configuration, with RF switching realized through a passive power dividing/combining network to eliminate the locking risk of conventional RF switches. Without significantly increasing size, weight, or power consumption, this design substantially enhances module reliability, providing a high-reliability, low-cost solution to single-point failures in the driver stage. The design has been successfully validated in orbit, offering a practical engineering reference for future high-reliability space-borne SSPAs.