We present a theoretical and experimental study on sensitivity enhancement of a fiber-loop cavity ring-down pressure sensor. The cladding of the sensing fiber is etched in hydrofluoric acid solution to enhance its sensitivity. The experimental results demonstrate that the pressure applied on the sensing fiber is linearly proportional to the difference between the reciprocals of the ring-down time with and without pressure, and the relative sensitivity exponentially increases with decreasing the cladding diameter. When the sensing fiber is etched to $41.15\mathrm{\mu m}$ , its sensitivity is about 36 times that of nonetched fiber in the range of 0 to $32.5\mathrm{MPa}$ . The measured relative standard deviation of the ring-down time is about 0.15% and, correspondingly, the least detectable loss is about $0.00069\mathrm{dB}$ .