I use jekyll with github-pages and when input "##" i found there is a "#" before the title , i don't want it .
How can i hide the "#" ?
This depends on theme which is used by you.
The #
symbol is a hash-link that helps us to share link to specific section of you post.
If you don't want to see this, edit the source of your theme, or just change the theme you are using.