// 使用這種方式合并單元格時，要給單元格逐一設置帶邊框的樣式
sheet.addMergedRegion(new CellRangeAddress(1, 1, 1, 3));
// 例如：
HSSFCellStyle defaultStyle = ExportToExcelUtil.setDefaultStyle(workbook);
// 為單元格設置帶邊框的樣式
setCellStyles(row, 2, 3, defaultStyle );

/**
* 設置單元格樣式
*/
private void setCellStyles(HSSFRow row, int start, int end, HSSFCellStyle cellStyle) {
    for (int i = start; i <= end; i++) {
       cell = row.createCell(i);
       cell.setCellStyle(cellStyle);
    }
} 

/**
* 帶邊框的樣式+
*/
public static HSSFCellStyle setDefaultStyle(HSSFWorkbook workbook) {
	HSSFCellStyle cellStyle = workbook.createCellStyle();
	// 邊框
	cellStyle.setBorderBottom(BorderStyle.THIN);
	cellStyle.setBorderLeft(BorderStyle.THIN);
	cellStyle.setBorderRight(BorderStyle.THIN);
	cellStyle.setBorderTop(BorderStyle.THIN);
	return cellStyle;
}

// 合并單元格
CellRangeAddress region = new CellRangeAddress(1, 1, 2, 2);
sheet.addMergedRegion(region);
                        
// 合并之后為合并的單元格設置樣式
setRegionStyle(sheet, region, defaultStyle);

/**
* 為合并的單元格設置樣式（可根據(jù)需要自行調(diào)整）
*/
@SuppressWarnings("deprecation")
public static void setRegionStyle(HSSFSheet sheet, CellRangeAddress region, HSSFCellStyle cs) {
    for (int i = region.getFirstRow(); i <= region.getLastRow(); i++) {
         HSSFRow row = sheet.getRow(i);
         if (null == row) row = sheet.createRow(i);
         for (int j = region.getFirstColumn(); j <= region.getLastColumn(); j++) {
              HSSFCell cell = row.getCell(j);
              if (null == cell) cell = row.createCell(j);
              cell.setCellStyle(cs);
         }
     }
}