The invention relates to a device and a method for measuring the higher harmonic of a neutron monochromator. The device comprises a reactor, a neutron transmission channel, a neutron monochromator, a time-of-flight apparatus, a deflection monochromator and a rotary platform. By means of the deflection monochromator, the neutron wavelengths, the wavelength bandwidths and the percentage compositions of the primary and higher harmonics of a deflected beam can be measured. In combination of the result of the deflection monochromator with appropriate data processing, the neutron wavelength, the wavelength bandwidth and the percentage composition of the higher harmonic of the to-be-measured neutron monochromator, obtained in the deflected emerging beam, are ensured to be accurate. The defects that the space around the emerging beam is too narrow to place any TOF device and the wavelength bandwidth is influenced by the intrinsic error of the TOF device can be overcome.