For an accurate prediction of carbon and oxygen impurities in multicrystalline silicon material for solar cells, global simulation of coupled oxygen and carbon transport in a unidirectional solidification furnace was implemented. Both the gas flow and silicon melt flow were considered. Five chemical reactions were included during the transportation of impurities. The simulation results agreed well with experimental data. The effects of flow rate and pressure on the impurities were examined. An increase in the flow rate can reduce both carbon and oxygen impurities in the crystal, though the reduction of carbon is more obvious. An increase in gas pressure can also obviously reduce the oxygen impurity but has only a small effect on the carbon impurity.