In lightning protection, the lightning protection box is widely used in all kinds of low voltage distribution systems. To investigate the second induction under 8/20 μs impulse current, a great number of experiments based on the Haefely DIAS 733 testing system are carried out. Based on the electromagnetic induction theory, the voltage is calculated and the inductive loop is built in PSPICE. The results indicate: (1) When the design is one port with I/O and parallel wired, the secondinduction voltage amplitude obeys the laws of electromagnetic induction, and can be calculated with higher precision. (2) There are three methods (twintwisted, ground and input line nonparallel, dividual ports of I/O and nonparallel designs) to reduce the electromagnetic induction effectively, and get a lower residual voltage. (3) Experiments show that when the internal wiring scheme adopts the dividual ports of I/O and nonparallel design, the effect is the most obvious. The residual voltage has about 45% reduction at the same sampling points under 60 kA impulse compared with before.