Aiming at the requirement of long-focal-depth elements used for the plasma diagnosis in inertial confinement fusion (ICF), a spheric lens with 150 mm aperture, 450 mm focus length, and 5 mm depth of focus is designed by nonlinear phase fitting method. In order to verify the correctness and feasibility of this method, a lens with a long-focal-length and the same F numbers (calibre 80 mm, focal length 240 mm) is fabricated. Moreover, the on-axis and transverse intensity distributions of parallel light after passing the lens are measured. The theoretical and experimental result indicates that the beam from the long-focal-depth lens has long focal depth, small focal spot and side lobe and it can meet the requirements of inertial confinement fusion.