Texit is only available in mac
Lyx is too complicated to setup,need to install several softwares.
Is there a simple solution?
TeX and LaTeX carry a fair piece of complexity with them (being Turing complete programing languages), so it is hard to have both "simple" and "complete". But most users want or need "complete", so you don't find many "simple" options.
Depending on your needs, the answer to the Stack Overflow question LaTeX: Are there any web frontends available? (and various related links) might help.
Yes. You can use emacs plugin preview-latex to do this. However, this means using emacs, which may or may not be suitable. (Admittedly, calling emacs simple is a stretch.)
See similar question on StackOverflow.com