An S-band MILO was designed by 2D particle-in-cell(PIC) electromagnetic code, based on the L-band hard-tube MILO of the Philips laboratory. The working frequency of the S-band MILO was 2.3 GHz. The design was further improved to increase the output power and efficiency. The emission increase from the cross section resulted in better magnetic insulation. A metal film was used to replace the conductor so that the load current, which had only supplied magnetic insulation, could be used to participate in the wave-beam energy transformation. The average output power in a period was 2 GW when the input voltage was 450 kV and the input current was about 45 kA. The efficiency was up to 10%.