The invention discloses a gradient amplifier and an NMR imaging device. The gradient amplifier applied to the NMR imaging device provides a current for a gradient coil in the NMR imaging device. The gradient amplifier comprises a rectifier, an H bridge, a filtering circuit and a control circuit; the rectifier is connected with an AC power supply, and rectifies AC provided by the AC power supply into DC; the input end of the H bridge is connected with the output end of the rectifier, and the output end of the H bridge is connected with the filter circuit; the filter circuit filters harmonic waves from output current of the H bridge; the output end of the filter circuit is connected with the gradient coil; the filter circuit comprises a filter inductor and a variable filtering impedor in serial connection; and the control circuit increase the impedance of the variable filtering impedor when the given current of the gradient coil is greater than or equivalent to a saturated current reference value of the gradient coil. Thus, current oscillation, caused by saturation of the filter inductor, of the gradient coil can be avoided, the imaging quality is improved, and the manufacture cost of products can be reduced.