Hello Suyan Dong,
You should attach to ColumnNotify event and cancel it when NotifyCode is ColumnSortingNotifyCode for the desired column:
nTreeList1.ColumnNotify += nTreeList1_ColumnNotify;
...
void nTreeList1_ColumnNotify(object sender, NTreeListColumnNotifyData data)
{
if (data.Column == nTreeList1.Columns["Project"] && data.NotifyCode == NTreeList.ColumnSortingNotifyCode)
{
data.Cancel = true;
}
}