After 2 days works again, I think I've looked out the cause of floppy
booting fail on IBM Thinkpad 755CX Laptop.
 
My IBM TP755CX's floppy driver is 2.88M, but the install floppy will
normally be 1.44M. 

I then traced codes in 'sys/arch/i386/stand/lib', and found the 
bios_disk.S using BIOS INT 0x13 (function 0x08) to get the geometry 
of floppy, but the INT 0x13 only return the geometry of *floppy driver*
(which may be different from the floopy disk insert in it).

In my case, the get_diskinfo() in bios_disk.S return the geometry of
my floopy driver is:
   max cylinders:     79
   max sectors/track: 36
as a 2.88M floppy driver,
then set_geometry(&d) in bootsectmain.c(../stand/lib/crt/bootsect/) use
this geometry and the rest of codes read sectors using this geometry
whick cause booing procedure fail.

I manully force the get_diskinfo() in bios_disk.S return the floopy
geometry:
   max cylinders:     79
   max sectors/track: 18
as a 1.44M floppy driver,
and NOW I boot NetBSD install floppy successfully !

To solve these problems, I think perhaps we should get disk geometry
from the bootsect itself, just as DOS and other *BSDs do it. But to 
a same bootsect for both floppy disk and hard disk, we should think 
more.

Regards,
Deng.