This paper expounds the problem on elgenvalue reduced-order for flexible umltibody system model , and derives the elgenvalue modal reduced-order oriterion and calculation steps based on Riccati iteration algorithm in accordance with attitude dynamics of flexible spacecraft system. Analysis results indicate that it can reflect better the basic characteristics of the original system than usually eigenvalue truncation criterion and exactly preserve the lower frequencles and mode shapes of the original system. Dynamic response curves of reduced-order model accord better with original system.