Abstract: The traditional mode superposition scheme seems not so suitable while applied to dynamic analysis of spatial lattice structures. The reason that accounts for this inapplicability is analyzed and summarized in this paper. Based on deep understanding of free vibration mode characteristics and the high computational efficiency of LDR method, a modified mode superposition method, named Eigen-LDR method, is proposed. The computational method is programmed with Matlab. And the dynamic responses of 2 typical spatial lattice structures are computed with the new method. The numerical results show that the proposed method is effective, accurate and with high efficiency.