BACKGROUND: Recent studies suggest that radiotherapy at ultrahigh dose rates (>40 Gy/s, FLASH) offers normal tissue sparing effects while maintaining tumor control. There is significant interest in preclinical studies investigating the mechanism of FLASH sparing effects. PURPOSE: This study aims to commission a fixed proton beamline within a synchrotron-based proton therapy system for preclinical proton FLASH research. METHODS: Modifications were made to the Hitachi PROBEAT-CR synchrotron system to enhance RF extraction power and increase proton beam current at 142.4 MeV. A high-speed electrometer and an optimized transmission ion chamber (IC) were implemented for ultra-high dose rate (UHDR) beam monitoring and delivery, replacing the conventional beam monitoring IC. Beam output was measured using a Faraday cup in both UHDR and clinical modes. Gafchromic film measurements and Monte Carlo simulations were employed to validate dose delivery in a solid water phantom with various spot scanning patterns. RESULTS: The calibration of transmission IC against Faraday cup shows sufficient charge collection efficiency at both clinical dose rates and UHDR. The UHDR PBS beamline demonstrates better than 1% reproducibility and linearity in the absolute beam output. Due to the limited charge per spill, the delivered dose per spill is inversely proportional to the field size. However, the system can deliver up to 41.4 Gy (268.1 Gy/sec) at 2 cm depth with a field size (FWHM) of 8.2 mm, demonstrating suitability for small animal proton FLASH irradiation studies. CONCLUSION: We successfully commissioned a fixed beam proton UHDR PBS beamline in a synchrotron-based proton therapy system. Despite synchrotron-specific system constraints, our system enables controlled UHDR delivery for preclinical proton FLASH research.