Using OptiStruct as the tool of optimization, the topology optimization of one special carriage’s square cabin is studied. The methodology of structural topology optimization is present and the key technology is discussed. the weight of the top-placed facility pedestal is set as the optimization objective,and its optimization model is developed. The establishment method of the optimization objective function, design variables and constraint condition are introduced. Through optimization calculation, the result of topology optimization is analyzed. The conclusion provides a significant researching basis for lightening the weight of cab.