Based on the Ronchi-Talbot effect and moire fringe technique, the focal length measurement method using divergent light and unequal-period grating is used for the measurement of the focal length of lens with a small aperture and long focal length. The long focal length of a plano-concave lens as a lens to be measured with a processing error in the curvature radius is measured. When the true curvature radius of the lens is unknown, first, the influence of the curvature radius error (of the lens to be measured) on the focal length detection accuracy is analyzed, and the position of the lens in the whole detection system is determined. Then, the focal length of the lens is measured, and multiple groups of measured focal length values are calculated separately. By comparison, it can be found that when the curvature radius remains unknown, the repeatability and stability of the focal length measurement are consistent. The measured focal lengths are in the range of 33200--33270 mm. The repeatability precision is greater than 0.055%, and measurement accuracy is better than one-fifth of the focal depth. These results prove that the proposed method is reliable and efficient in the focal length measurement of small-aperture lenses with long focal length.