Abstract: To prevent cascading failures, safety margin and voltage stability of power grid in operation must be fully considered．Here we propose to convert difference between injected power of node in current operating state of grid and injected power of node in critical state into a safety margin index．We propose to combine minimum eigenvalue index of Jacobian matrix (to measure overall voltage stability of power grid) and L index (to measure voltage stability of a node in power grid), to give a comprehensive voltage stability index．Various constraints are given, in combination with actual situation of power grid, thereby determining a complete cascading failure prevention model．The optimization process should be monitored,to see whether effects of power flow transfer in remaining branchs after the initial failure of power grid are decreasing．The cascading failure prevention model proposed here is proved to be effective．This paper verifies the feasibility and effectiveness of the constructed model for preventing cascading failures by simulation of IEEE39 node system．