如果thead中需要合并列
#tblDataList thead {display:block;}
#tblDataList tbody { display:block;height:600px; overflow-y:scroll;overflow-x: hidden;}
#tblDataList thead tr { display:table; width:100%; table-layout:fixed;}
#tblDataList tbody tr {display:table; width:100%; table-layout:fixed;}
#tblDataList thead {width: calc( 100% - 1.17em )}
.ul_inth{ padding: 0px; margin: 0px; border-top: 1px solid;}
.li_inthF{width: 33%;list-style: none;float: left;}
.li_inth{ width: 33%;list-style: none;float: left; border-left: 1px solid;}
<table id="tblDataList" cellpadding="0" cellspacing="0">
<thead>
<tr>
<th rowspan="2">时间</th>
<th colspan="3">烟尘<ul class="ul_inth"><li class="li_inthF">实测(mg/m3)</li><li class="li_inth">折算(mg/m3)</li><li class="li_inth">排放量(mg/m3)</li></ul></th>
<th colspan="3">氮氧化物<ul class="ul_inth"><li class="li_inthF">实测(mg/m3)</li><li class="li_inth">折算(mg/m3)</li><li class="li_inth">排放量(mg/m3)</li></ul></th>
<th colspan="3">二氧化硫<ul class="ul_inth"><li class="li_inthF">实测(mg/m3)</li><li class="li_inth">折算(mg/m3)</li><li class="li_inth">排放量(mg/m3)</li></ul></th>
<th rowspan="2">气压<br>(mbar)</th>
<th rowspan="2">烟温度<br>(℃)</th>
<th rowspan="2">烟流速<br>(m/s)</th>
<th rowspan="2">氧<br>(%)</th>
<th rowspan="2">标态流量<br>(m3/s)</th></tr></thead>
<tbody>
<tr class=" "><td>2016-01-01 00:00:00</td><td>12.9900</td><td>16.3800</td><td>0.0487</td><td>38.8600</td><td>48.9800</td><td>0.1457</td><td>10.7600</td><td>13.5600</td><td>0.0403</td><td>-0.0700</td><td>54.4300</td><td>6.3500</td><td>18.6200</td><td>62.4700</td></tr>
<tr style=""><td>2016-01-01 00:05:00</td><td>12.8000</td><td>16.1500</td><td>0.0480</td><td>38.6700</td><td>48.8000</td><td>0.1450</td><td>10.4700</td><td>13.2100</td><td>0.0393</td><td>-0.0700</td><td>54.4000</td><td>6.3500</td><td>18.6200</td><td>62.5100</td></tr>
</tbody></table>