1.Install-Package NPOI -v 2.4.0 

2.

using NPOI.XSSF;
using NPOI.XSSF.UserModel;
using NPOI.SS.UserModel;
using System.IO; 
static void ExportDataTable(DataTable dt)
        {
            string exportedExcelFullName = Directory.GetCurrentDirectory() + "//" + DateTime.Now.ToString("yyyyMMddHHmmssffff") + ".xlsx";
            if(dt!=null && dt.Rows.Count>0)
            {
                XSSFWorkbook workBook = new XSSFWorkbook();
                ISheet firstSheet = workBook.CreateSheet("First Sheet");
                IRow headerRow = firstSheet.CreateRow(0);
               
                for(int i=0;i<dt.Columns.Count;i++)
                {                    
                    ICell headerCell = headerRow.CreateCell(i);
                    headerCell.SetCellValue(dt.Columns[i].ColumnName?.ToString());                     
                }                

                for (int i = 0; i < dt.Rows.Count; i++)
                {
                    IRow dataRow = firstSheet.CreateRow(i + 1);
                    for(int j=0;j<dt.Columns.Count;j++)
                    {
                        ICell dataCell = dataRow.CreateCell(j);                        
                        dataCell.SetCellValue(dt.Rows[i][j]?.ToString());                        
                    }
                }
                for(int i=0;i<dt.Columns.Count;i++)
                {
                    firstSheet.AutoSizeColumn(i);
                }                

                using (FileStream excelStream = File.Create(exportedExcelFullName))
                {
                    workBook.Write(excelStream);
                } 
            }           
        }

3.让Excel单元格自适应单元格大小

for(int i=0;i<dt.Columns.Count;i++)
{
firstSheet.AutoSizeColumn(i);
}

最后效果：