The narrowband filter with central wavelength of 121.6 nm is designed based on the double half-wave Fabry-Pérot interference filter, with peak transmittance of 6.783% and half-peak bandwidth is 10.7 nm. A filter with the peak wavelength of 217 nm is designed, prepared and measured, and optical constants and scale of film thickness are precise. The filter with central wavelength of 121.6 nm is prepared based on the 217 nm filter,the transmittance of filter is measured with the central wavelength of 120.74 nm, the peak transmittance of 5.94%, the half-peak bandwidth of 12 nm. The prepared filter is in accordance with prediction design in advance generally. At last the difference between experimental value and theoretical value is analysed.