Abstract: The paper concerns in the idea and method on optimizing solenoid lens design. The use of ferromagnetic circuit can increase κ times of magnetic field of solenoid lens, and the value of κ can be determined by the geometrical parameter (r_(z),t_(z)) of solenoid lens definitely. A 3-D κ-curved surface was set up with data calculated by Poisson code, and a fitting formula of κ-curved surface was found, which has good accuracy in common use range. Furthermore database file SLD-Y supported κ-curved surface was programmed in order to find κ as known r_(z) and t_(z). Now the database file SLD-Y has been used for our solenoid lens design successfully.