Voltage stability is one of the important parts for the secure and stable operation in power systems. However, some credible contingencies may cause the load margin significantly reduced to a lower level and even cause voltage instability. To increase the voltage stability, the bus operation mode is applied to optimize the grid network topology to enhance the load margin of power system and ensure the voltage stability for base case and contingency cases. The contingency-constrained voltage stability preventive control model is proposed, and a stage-based bus operation mode optimization methodology is developed in this paper. To reduce the computation complexity, the sensitivity of line parameters is employed to pre-screen the candidate set in which the bus operation schemes can enhance the load margin, and the effective ones are ranked by the look-ahead margin method. Finally, the continuation power flow method is used to identify the ‘best’ bus operation schemes. Numerical studies on the IEEE 118-bus power system and 1648-bus power system demonstrate the effectiveness of the proposed methodology.