Disclosed are a device and method for measuring the focal length of a large-aperture telephoto optical lens. The device includes an interferometer, a standard spherical reflector, a photoelectric auto-collimator, a laser tracker, and a data processing unit. A test light path is adjusted so that the optical axis of an optical lens is parallel to the parallel light emitted by the interferometer. Thesmall-aperture standard spherical reflector is placed at the theoretical position of an image focal point, the standard spherical reflector is adjusted to the actual position of the image focal point, and the laser tracker measures the spatial position of the standard spherical reflector. The interferometer is deflected, the deflection angle is measured, the standard spherical reflector is adjusted to the actual position of the image focal point, and the spatial position of the standard spherical reflector is measured with the laser tracker. The data processing unit calculates the focal length of the optical lens based on the deflection angle of the interferometer and the amount of change in the spatial position of the standard spherical reflector. The device and method in the invention can be used for focal length measurement of large-aperture telephoto optical lenses with different wavelengths, and have very important applications in the installation and testing of space optical remote sensors.