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