It was a long story, but in short, follwing is what I have done to get the current problem:
0. I was using Slackware 12.2 on Eeepc;
1. Somehow, I tried to upgrade KDE3.5 to KDE4. So I used 'slackpkg update' and 'slackpkg install kde', connecting to one mirror server of Slackware 13.1;
2. *Before* the upgrade work was done, I (forgot and) shut down the machine;
3. Then I could not start the X window (and immediately realised my mistake), there was some 'no screen' error; I tried to upgrade the KDE again, this time the task was completed, but startx gives only a dark blue screen with nothing on it;
4. I googled for a while, getting no answer. I decided to upgrade the whole system to 13.1(from 12.2). I used 'slackpkg upgrade-all', but there were many 'package not installed, gpg error!' things;
5. I learned that it is necessary to read 'UPGRADE.txt', I followed the one in the package directory of Slackware 13.0, it is for upgrading 12.2 to 13.0 (the one under package directory 13.1 is not more relevant, I guess, since it is for 13.0 to 13.1). This time, upgrade process was of no problem.
6. Then I startx, this time, x window starts, I can even hear the music, but there was nothing but some colored strips, the cursor now turns to be some points.

I hope I have stated the process clearly, now, any idea for the problem? Thanks!

It's quite possible that your OS is a mix of 13.0 and 13.1 packages (the latter having been installed during your first attempt at a direct upgrade from 12.2 to 13.1).

I would strongly recommend that you do a clean install (download the DVD or the first 3 CDs). Of course , do not forget to backup your data

However if you think that your upgrade to 13.0 is correct, then have a look at file CHANGES_AND_HINTS.TXT for any pointer to your problem (This file is a must-read).

I'm not a slackpkg buff but you should upgrade slackpkg *first* -- I believe that would have cured the gpg errors (though I'm not positive). Second, you cannot use KDE4 from 13.0 or 13.1 with 12.2 -- it is missing a lot of libraries I would imagine and was compiled against newer versions of libraries than are included with 12.2. There is a pretty big difference between the two and KDE isn't very flexible with dynamic library changes post-compilation.

Unless you know what you are doing (and it doesn't look like you do when it comes to upgrading...) you should upgrade to 13.0 and then to 13.1, though at this point a fresh install is probably easier. If you do want to upgrade you should follow UPGRADE.TXT for 13.0 and either use upgradepkg --reinstall or use the equivalent option in slackpkg...and then remove any packages from 13.1 that were installed that will not be overwritten by 13.0 (and also remove obsolete packages from 12.2). slackpkg should be able to create a list of packages that are not included in the default installation...so assuming you are using a 13.0 mirror and have reinstalled all of the 13.0 packages (following UPGRADE.TXT for 13.0) it will list all non-13.0 packages (INCLUDING third-party packages). You would want to at least remove the 12.2/13.1 packages listed. slackpkg clean-system would remove *all* non-13.0 packages but you may want to track or keep old third-party packages...again I don't know slackpkg very well but this command at least used to work:
Once you get 13.0 working properly then upgrade to 13.1 following the instructions in UPGRADE.TXT for 13.1.

You have a fun road ahead of you...it is probably easier to install fresh (make sure you make backups) but it is not impossible to recover your system -- though the help you get here will likely be confusing since it is hard to tell exactly what state your system is in.

1 members found this post helpful.

Thanks for your advice gegechris99, and T3slider!
I just come back after a clean installation of 13.1, now everything works fine(as it should be...)!