To overcome the shortcoming of transmission lines update simulation in the slow dynamic process of the existing OPA model， an improved OPA model based on transmission network expansion planning is proposed. This model not only considers that transmission lines are updated by increasing their capacities， but also adopts the method of transmission network expansion planning to rebuild lines to achieve grid upgrading so that the simulation of the slow dynamic process is in more accordance with the evolution of a real power system. Monte Carlo sampling method is used to obtain the outage state of each line in the fast dynamic period， and the simulation of the operation mode of each state is used to determine whether cascading failures and loss of load will happen. The simulation results indicate that this model can simulate self-organized criticality of grid. Comparing with an example， it clearly explains that the outage scale of failures is influenced by the rebuilt lines in the transmission network while simulating cascading failures. Using transmission network expansion planning approach to holistically upgrade power grid can effectively enhance the grid against risks of cascading failure.