Why the iphone ios version updated automatically ?
I have iphone 6s 32gb
IOS version 11.4.1
I have already turn off automatic update option but ios updates without any notifications about it
It didn't. You authorized it at some point. The updates only auto download, the do NOT auto install. The setting for automatic downloads that you turned off is for apps from the app store, not for iOS updates as that auto download feature CANNOT be turned off. Sorry.