The synthetic index of river-bed stability for alluvial rivers is used to judge the intensity of sediment movement and river-bed evolution caused by sand dune movement, and to distinguish river pettern as reference. The research findings of river dynamics show that the longitudinal river-bed stability has a relation with the longitudinal sediment transport capacity which is proportional to the longitudinal flow parameter. By analogy with the relation between the longitudinal river-bed stability and longitudinal flow parameter, the transverse river-bed stability is assumed to be proportional to the transverse flow parameter. The synthetic index of river-bed stability is determined by the longitudinal river*.bed stability in contrast to the transverse river*.bed stability. Through derivation the general formula is obtained, which reflects the flow dynamic factors, the characteristics of river-bed geometry and boundary. According to the data of the river pattern of natural and model rivers, the synthetic index is determined. This index reflects the river*.bed synthetic stability and can be applied to distinguish river patterns.