这个问题我的办法是，用户有个活跃时间间隔问题。超过这个活跃时间间隔的则可以认为是离线，后者可以登陆 ，如果无后者登陆，但并不是说前者就需要重新登陆了。因为他的session还在的。只能是这个时间设置的越小越精确，也是不能做到绝对的。
代码贴出来给楼主参考下。
Function CheckOnline()
DIM IP,rsPrv,Sql,PrvDbState
PrvDbState = False
If DBSTATE = False Then
DbOpen()
PrvDbState = True
End If
Set rsPrv=Server.CreateObject("ADODB.Recordset")
If Session("UserName") = "" then
Sql="select * from [Online] where SessionID='"& Session.Sessionid &"'"
rsPrv.Open Sql,Conn,1,3
If rsPrv.Eof then
rsPrv.AddNew
rsPrv("SessionID") = Session.SessionID
rsPrv("GroupChargeLv") = -1
rsPrv("LastActTime") = Now()
rsPrv("UserIP") = GetIP
rsPrv("OnLineTime") = 0
rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")
Else
rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")
rsPrv("OnLineTime") = rsPrv("OnLineTime") + DateDiff("n",rsPrv("LastActTime"),Now())
rsPrv("LastActTime") = Now()
End If
rsPrv.Update
rsPrv.Close()
'response.Write "notlogin"
Else
'response.Write "logined"
Sql="select * from [Online] where UserName='"& Session("UserName") &"'"
rsPrv.Open sql,Conn,1,3
If rsPrv.Eof then
rsPrv.AddNew
rsPrv("SessionID") = Session.SessionID
rsPrv("UserName") = Session("UserName")
rsPrv("GroupChargeLv") = Session("GroupChargeLv")
rsPrv("LastActTime") = Now()
rsPrv("OnLineTime") = 0
rsPrv("UserIP")= GetIP
rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")
Else
If rsPrv("SessionID") <> Session.SessionID And Application("LoginSet")(1) = False Then
InfoTo "LoginOut.asp","该帐户已在其他地方登陆，网站设置1个ID只能有1个登陆\n你可以稍候尝试登陆。"
Response.End()
End If
rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")
rsPrv("OnLineTime") = rsPrv("OnLineTime") + DateDiff("n",rsPrv("LastActTime"),Now())
rsPrv("LastActTime")=Now()
End If
rsPrv.Update
rsPrv.Close()
End If
Set rsPrv = Nothing
If DateDiff("s",Application("OnLineLastDelete"),Now()) > Int(Application("DELETEONLINEDIFF")) Then
Application.Lock()
Application("OnLineLastDelete") = now
Application.UnLock()
Conn.ExeCute("delete from [Online] where datedIff('s',LastActTime,Now())>"& Int(Application("CHECKONLINEDIFF") &"")) '删除x秒没有活动的访客
End If
If PrvDbState = True Then DbClose()
End Function