Abstract: The solar X-ray detector (SXD) onboard the Macau Science Satellite-1B was designed to monitor solar flare bursts and to study the solar activity in the 25th solar cycle. The SXD includes two parts: a soft X-ray detection unit and a hard X-ray detection unit. Both the soft X-ray detection unit and the hard X-ray detection unit include two collimators, two X-ray detectors (a silicon drift detector and a cadmium-zinc-telluride detector), and a processing circuit. Compared with similar instruments, the energy range of the SXD is wider (1–600 keV) and the energy resolution is better (150 eV at 5.9 keV, 12% at 59.5 keV, and 3% at 662 keV).