I do not know why, but Kate(http://kate-editor.org) really have a line length limitation, with the default value of 1024 characters. When a file containing a line with more then 1024 characters is opened, a prompting will show: "xxx was opened and contained too many long lines (more than 1024 characters). Too long lines were wrapped and the document is set to read-only mode, as saving will modify its content."
This can be changed using Kate Setting: Settings -> Configure Kate -> Open/Save -> Line Length Limit. Change this value to 0 will solve this problem.