I solved this problem by re-install the OS. When I install the OS, I did not 
check the checkboxes to install updates and third-party software (I checked to 
install third-party software last time). Therefore the nouveau driver instead 
of the NVIDIA one will be installed. Then I changed the apt source to a local 
software mirror, and use Ubuntu Drivers to install the NVIDIA driver. And 
everything now works fine.
  I think the problem may caused by incomplete install on the NVIDIA driver. 
Since I am in mainland China, the network may be the primary reason for this 
incomplete install.

During the above procedure, I found that the OS will become extremely slow in 
the following situation.
- Using nouveau driver.
- The laptop internal display is disabled (lip off or disabled in Display 
- The external monitor is the only display.
I did not investigate deeper and I quickly installed the NVIDIA driver.

Thanks for the help!

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  [nvidia][Lenovo P51] Mini-DP and HDMI ports are missing

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