装好Xubuntu后，一直存在着这个不能锁屏的问题，但是自己一直在忙，也懒的去弄这个事情。刚刚今天下班比较早，就来处理了一下。Google了一下，发现这是一个已经报告的bug,已经有了解决方案。

https://bugs.launchpad.net/ubuntu/+source/xfce4-utils/+bug/200665

Binary package hint: xfce4-utils
1) Ubuntu version: Xubuntu 8.04
2) Source package: xfce4-utils and xfce4-panel
3) Package version: 4.4.2-3ubuntu2
4) What you expected to happen:

The "Lock screen" button should lock the screen.
5) What happened instead:

It doesn't lock the screen.
6) Steps to reproduce:

1. Right click on the Panel -> Add New Item -> Action Buttons -> Add

2. Select action type: Lock screen

3. Click on the "Lock screen" button in the Panel.

4. Nothing happening.
   7) Possible reason:

The "Action Buttons" panel plugin uses the /usr/bin/xflock4 script (provided by the package xfce4-utils).

This script try to use xscreensaver to lock the screen ("xscreensaver-command -lock"), but if xscreensaver isn't running, try to use gnome-screensaver ("gnome-screensaver-command --lock"), but if gnome-screensaver isn't running, try to use the "xlock" command to lock the screen.

But, by default, neither xscreensaver, nor gnome-screensaver is running, and none of the packages providing "xlock" are installed (xlockmore and xlockmore-gl).
8) Possible solutions:

• Modify the xflock4 script, to show a warning, if xscreensaver or gnome-screensaver isn't running.
  

OR

• Make xfce4-panel depend on xlockmore or xlockmore-gl, so it will be able to lock the screen, even if xscreensaver or gnome-screensaver isn't running.
  [9) Another suggestion: May be good, to use the keyboard shortcut Ctrl+Alt+L to lock the screen. Currently Ctrl+Alt+Del is used, but it could be useful, to let lock the screen also with Ctrl+Alt+L as in GNOME and KDE.]

　　查了一下我的Xubuntu，发现没有安装xlock,下载xlockmore安装后，问题解决。xlockmore可以在这里下载：http://ftp.debian.org/debian/pool/main/x/xlockmore/xlockmore-gl5.22-1.4i386.deb