Abstract：Using systems theory，the structural relation of chain effects on natural disaster is mathematically analyzed；and a mathematical model is deduced. According to the analysis of the mathematical model of chain effects of natural disaster，some complex laws of chain effects natural disaster and an originality of the complexity are achieved；and it shows that chain effects on natural disaster are caused and dominated by both the environments and structural relation. The result shows that the structural relation chain-cycle is the main originality of natural disaster complexity. If there is a structural relation chain-cycle in natural disaster systems during evolution，the properties of natural disaster systems would reserve their memory，irreversibility，self-organizing，loading path effect，discontinuity and other variables. Application in slope of progressive failure demonstrates that the conclusion is practical.
