A hard x-ray spectrometer based on a φ25 × 25 mm2 LaBr3 scintillator has been developed to measure the runaway electron bremsstrahlung. Simulation results by GEANT4 show that x rays with an energy of 15 MeV can be effectively detected. In order to tolerate a high count rate, a self-made fast shaping amplifier is used to process the detector output into a fast bipolar pulse. The tested energy resolution, which was 3.4% for Cs-137 662 keV gamma rays, shows that the shaping amplifier does not significantly degrade the energy resolution. The results of the HL-2A discharge show that the maximum count rate can reach 2 MHz, and the time resolution of the spectrometer can reach the millisecond level.