The measurement and study of the Sun's magnetic field is an important branch of solar physics, most of the Sun's phenomena and processes are closely related to the magnetic of the Sun. Magnetograph is a solar narrowband observation system based on tunable filters. The magnetograph can measure the magnetic field and velocity field of the solar atmosphere with high accuracy and is a piece of important observation equipment for solar physics research. Due to processing and assembly errors and incidence angles in different fields of view, the central wavelength of the solar narrowband observation system will deviate from the calibrated wavelength, which is called wavelength drift. The wavelength drift of the observation system seriously affects the accuracy of the magnetic field and velocity field measurement. The wavelength drift must be measured accurately and corrected during the measurement of the magnetic field and velocity field. The wavelength drift is different for different fields of view. To achieve high-precision measurement of the magnetic field and velocity field, it is necessary to precisely measure the wavelength drift in the full field of view.Aiming at the wavelength drift problem of the narrowband solar observation system based on the Lyot filter of ground-based large aperture solar telescope, this paper proposes an online full-field wavelength drift measurement method. This method can realize the high-precision measurement of wavelength drifts in the full field of view without changing the structure of the observation system. Online measurement of system wavelength drift refers to scanning the system spectral lines based on the existing configuration of the observation system to obtain spectral profiles at each field of view. The line center of the field of view can be obtained by fitting the spectral profiles of different fields. The method of obtaining system wavelength by spectral profile scanning is an algorithm based on the light intensity. The fluctuation of light intensity at one field of view will affect the wavelength drift measurement here. The structure of the solar atmosphere and the change of atmospheric transmittance are two important reasons for the fluctuation of light intensity in data acquisition. The spatial structure of the solar atmosphere will shake irregularly in the field of view due to telescope tracking errors and atmospheric turbulence. The method in this paper can eliminate the light intensity error caused by these factors and improve the wavelength drift measurement accuracy. Firstly, control the telescope to point to different positions in the quiet area of the solar disk center, and acquire the data of each wavelength by flat field mode. The average of multi frame images can smooth out the solar atmospheric structure and reduce the error caused by these structures. Secondly, synchronous acquisition of light intensity of broadband channels can correct narrowband image intensity frame by frame. Finally, the scanning spectral profile will be obtained for each field of view. The Gaussian fitting method can obtain the wavelength drift result from the scanning spectral profile.We have carried out the experimental verification in the high-resolution solar photosphere observation system of the 1 m New Vacuum Solar Telescope (NVST) in the Fuxian Solar Observatory (FSO). The field of view of the experimental system is 102″. We observed 11 wavelength points of ±6 pm centered on 5 324.191 nm. And we acquired 10 images at each wavelength point 15 times. The experimental results show that: in the 102″ field of view, the static error amplitude of full field wavelength drift is 1.6 pm; the wavelength drift distribution in the field of view is close to the sphere; the mean wavelength drift redshifts gradually throughout the day. At the same time, through many times of measurements, the measurement error RMS of the online measurement method for wavelength drift proposed in this paper is less than 0.1 pm.The online measurement method of wavelength drift proposed in this paper realizes high-precision measurement of wavelength drift. The high-precision measurement of wavelength drift improves the measurement accuracy of the solar magnetic field and velocity field. The residual of spherical fitting of wavelength drift is less than 0.2 pm. It's a static error, which caused by the incident angle of the filter. The redshift of the mean wavelength drift comes from the Earth's rotation, it's a dynamic error. There are some ripples in the frequency drift result, the amplitude is 0.25 pm. They are caused by fringes on monochromatic images. Fringes on monochromatic images originate from the interference of monochromatic light passing through the filter at the detector window. Due to the limited accuracy of using the image processing method to remove ripples, it is better to use a window with a wedge angle in the system design to avoid interference. In addition, in order to improve the accuracy of full field wavelength drift measurement, it is also necessary to consider how to eliminate the influence of dirty spots in the observation system.