Abstract: The star tracker installed as an attitude measuring device on the surface of the GEO satellite is susceptible to the space charging environment and is likely to become abnormal in the orbit under the interference of strong electromagnetic pulses. In this paper, the coupling effect of the star tracker structure under a strong electromagnetic field is studied by numerical simulations and measurement tests. The electromagnetic shielding and the coupling field distributions in the star tracker under a strong electromagnetic field are simulated, with a corresponding experiment to validate the shielding effectiveness. It is indicated that the high frequency part of the strong electromagnetic field will induce a strong electromagnetic field when it is coupled with the star tracker through its optical window. At the resonance frequency point, the field strength will be enhanced, to significantly reduce the shielding effectiveness of the structure and to interfere with the internal electronic devices. Thus the protective countermeasures need to be taken. The results of this paper can provide some guidance for protecting the star tracker under a strong electromagnetic environment and some help for the on-orbit anomaly analysis.