Couldn't shift to the lowest(the 1st)  speed of the back cassette. I knew it was just a minor problem. But couldn't help thinking this sometime.
Without the manual, the repairing took me some pretty time. Eventually, thanks to the mighty google, I found some instructions on the internet and the reparing was really sped up.
On the rear deraileur,the screw marked "H" means it is for High gear adjustment, while the screw marked with "L" is for low gear adjustment.
the high gear limit screw stops the derailleur  from shifting past high gear (smallest sprocket), and the low gear screw prevents it  from going past low gear (largest sprocket). If a limit screw is too loose, the  chain will fall off. If too tight, the system will not engage the  gear.
more details can be found on the following web site.
http://www.bikewebsite.com/rearder.htm