A two-stage magnetic compression pulse generator which can output fast forward nanosecond high voltage narrow pulse was designed for underwater discharge. According to the principle of magnetic pulse compression, the parameters of magnetic switch were designed. Breaking the water medium to generate a plasma discharge channel requires a uniform electric field, so the external magnetic reset circuit cleans up the remanence. The iron-based nanocrystalline core was selected in the PSPICE simulation software to create a magnetic switch model and the encapsulation library was added. The magnetic switch model can well simulate the working state of voltage and current staged compression of MPC (magnetic pulse compression) in practical applications. With the advantages of adjustable frequency, pulse width and phase, IGBT (insulated gate bipolar transistor) is often used as a primary control switch in practical applications. It is used to assist MPC in designing a packaging switch model of a transistor voltage reverse amplification delay circuit to achieve the correspondence with the actual selection of IGBT switch parameters. The simulation results show that a high voltage narrow pulse with a rising edge of 90 ns and an amplitude of 8.5 kV is output on the 50 Ω resistor, and the rising edge compression factor is 6.4.