Based on the theory of complex systems, and with means of Agent simulation technology, the critical evolution SLO(steady-latent-outbreak state) model and a related simulation model of single emergency is proposed. Research results show that the internal cause of single emergency is the accumulation of internal energy; the evolution is a process from the continuous accumulation of energy to its mass release suddenly. This kind of evolution inevitability could be changed by adjusting inside energy of the system itself or preventing internal transmission action. As the internal transmission action or transfer possibilities are equal to zero, single emergencies could be totally prevented.