How to paste source code to vim without error format?

Automatic indenting kicked in.

The easiest way to disable it is: :set paste

:help paste

'paste'                 boolean (default off)      
                        global
                        {not in Vi}
    Put Vim in Paste mode.  This is useful if you want to cut or copy
    some text from one window and paste it in Vim.  This will avoid
    unexpected effects.
    Setting this option is useful when using Vim in a terminal, where Vim
    cannot distinguish between typed text and pasted text.  In the GUI, Vim
    knows about pasting and will mostly do the right thing without 'paste'
    being set.  The same is true for a terminal where Vim handles the
    mouse clicks itself.