.checkboxlist label
{
margin-right: 20px;
}

var label;
$("#ddlplatform input:checkbox:checked").each(function () {
label += $(this).next().html();
});

<asp:CheckBoxList ID="ddlplatform" runat="server" RepeatDirection="Horizontal" RepeatLayout="Flow"
CssClass="checkboxlist">
<asp:ListItem Value="APP">APP</asp:ListItem>
<asp:ListItem Value="移動觸屏">移動觸屏</asp:ListItem>
<asp:ListItem Value="PC">PC</asp:ListItem>
<asp:ListItem Value="微信">微信</asp:ListItem>
<asp:ListItem Value="微博">微博</asp:ListItem>
</asp:CheckBoxList>

//使用平臺
string Platfrom = string.Empty;
foreach (ListItem li in this.ddlplatform.Items)
{
if (li.Selected)
{
Platfrom += li.Text + ",";
}
}
Platfrom = Platfrom.TrimEnd(',');