A microwave photonics fiber loop ring-down system is demonstrated in this paper. In comparison with the traditional time domain fiber loop ring-down setup, the demonstrated system is based on pure frequency domain measurement from a microwave-photonic configuration. The system consists of a direct-modulation laser with its modulation frequency scanned by a vector network analyzer. The amplitude and phase spectra of the demonstrated fiber loop ring-down system are then recorded, followed by a complex Fourier transform to acquire the ring-down curve in the time domain. The loss of the fiber loop can then be evaluated based on the calculated ring-down curve. The system delivers high signal-to-noise ratio, and averaging is not required compared with the traditional time domain measurement. The system is also insensitive to the sources of environmental noise. The measurement principle, experimental setup, and the mathematical model of the system are discussed in this paper. A proof-of-concept macrobending loss test is demonstrated