Jitao,
You did not mention which development environment you are using, so I will tell you which VI to use in LabVIEW and which NI-DAQ function to call. In LabVIEW, you will want to use Set SCXI Information.vi. This VI is located in Functions >> Data Acquisition >> Calibration and Configuration. If you are using NI-DAQ function calls, you will want to use SCXI_Set_Config().
Good luck with your application.
Spencer S.