thx for the reply Kovid
your program is indeed very cool
I finally solved the issue via everyone's help
as expected
$ xdg-open file1.pdf
opens file1.pdf using xpdf
after I customise it with
$ xdg-mime default evince.desktop application/pdf
it doesn't help at all
then I figured that
$ gnome-open file1.pdf
did the task I want
finally, I add "export DE=gnome" to ~/.bashrc
the problem solved
thx guys!