您很可能在.vimrc中设置了gdefault.
来自：help gdefault：
When on,the ":substitute" flag 'g' is default on. This means that all matches in a line are substituted instead of one. When a 'g' flag is given to a ":substitute" command,this will toggle the substitution of all or one match.
如果你没有在配置中设置它,你可以通过发出：verbose set gdefault？来查看它的设置位置.
