If I press CtrlC, jjj(select all lines) , Shifti, \, only first line changed.
Notice: Don't want to use :s substitute command.
The short answer is: In Visual Block mode select lines to their ends (
$) and use
A to append some text at the end of each line.
The longer answer:
ggto go the beginning of the first line.
CTRL-V(or the proper alternative
CTRL-Qin e.g. gVim) to start blockwise visual selection.
jjj$select all four lines down (
jjj) and to the right until their individual ends (
Astart appending some text ('
\' in your case).
In fact this is not exactly your desired result, but functually equal and achievable with a minimum effort and time:
short line \ very long line \ very very long line \ short line \