Introduction  The power of offshore oilfield group mainly comes from generating units， which has problems of poor power supply reliability， low power generation efficiency， high pressure of energy saving and emission reduction， etc.

  Method  According to the function orientation and working principle， the offshore step-down substation could be divided into six areas： 220 kV switchgear area， transformer area， 35 kV switchgear area， 10 kV switchgear area， reactive power compensation area and auxiliary room. The recommended layout plan was given through the study of each area.

  Result  The layout plan proposed in this paper plays an important role in improving the reliability of offshore step-down substation， ensuring the convenience of equipment maintenance and service， and minimizing the platform size.

  Conclusion  The layout plan proposed in this paper can play a good role in guiding offshore step-down substations in oilfield group project and has high reference value.