Since upgraded to 2.6.2, I found that the TexStudio cannot recognize my command path any more. I don't know if this is a problem of the software or my computer system. So, I would just leave this puzzle here, and to see if anyone else got the same problem.

Found of the problem: when I compile any tex  file, I got an error that "TexStudio Error: Could not start the command: pdflatex...". I found that I used "txs:///pdflatex" in my build option, and "pdflatex" in the command option of the configuration of TexStudio. If I replace the "pdflatex" with the full path of the installation direction, such as "E:/MikTex/miktex/bin/x64/pdflatex", then the compilation works like charm. Similar issue occurs on "bibtex" and other commands. So, I conclude that the TexStudio cannot recognize my installation direction of the MikTex in my computer, and hence cannot find the compiling commands in my case... I don't know if this is common for others or not.

TexStudio 2.6.4 beta also has the same problem for me.