废话不多说，直接上代码，此方法需要在网页初始化加载链接之前就行调用。

-(void)setChangUserAgent:(NSString*)userAgent{

    NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:userAgent, @"UserAgent", nil];

    [[NSUserDefaults standardUserDefaults]

     registerDefaults:dictionary];

    [[NSUserDefaults standardUserDefaults] synchronize];

    [self.view insertSubview:self.webView atIndex:0];

    if(@available(iOS11.0, *)) {

        self.webView.scrollView.contentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentNever;

    }else{

        self.automaticallyAdjustsScrollViewInsets=NO;

        // Fallback on earlier version

    }

}

调用如下

 [self setChangUserAgent:@"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.108 Safari/537.36 iphoneapp"];