The energy spectra of ion beams emitted from gas-puff Z-pinch plasma was experim entally studied with a compact Thomson spectrometer, which is composed of an inp ut window, deflecting electromagnetic fields,a rear pinhole, and a CR-39 track d etector. Ion beams were collimated by a front pinhole and the input window of Th omson spectrometer and deflected in the electromagnetic fields, and then they pa ssed through the rear pinhole to hit the CR-39 detector. Based on the detected p ictures of parabolas, we have calculated the energy spectra of ion beams and fou nd that the energy spectra move toward the direction of lower energy for a bad Z -pinch shot. A “bad Z-pinch shot" here means a shallower dip on the di/dt signal, which is the time derivative of Z-pinch current.