In order to avoid the adverse effects of modal transition and unreasonable mode selection on the accuracy of damage assessment results，a seismic damage assessment method for spatial grid structures considering the contribution of multiple modes was proposed． Firstly，based on the idea of modal pushover analysis and the structural dynamic theory，the equivalent static displacement expression corresponding to the multi-modal contribution under unit seismic force was obtained． Then，according to the difference between the equivalent static displacement of original structure and damaged structure，the structural characterization parameter of the earthquake damage degree， namely，the overall damage parameter，was generated． Finally，the state of structural damage and performance degradation was assessed based on the magnitude and change of the overall damage parameters of the structure． The numerical analysis results of single-layer reticulated shell model show that the overall damage parameter of the structure calculated according to the method in this paper can more accurately assess the seismic damage degree of the space grid structure based on considering the multi-mode modal contributions． The obtained results are in good agreement with the comparison of structural characteristic response． In addition，the accuracy of the assessment results can be guaranteed in the case of severe damages．