方法一：

AppDelegate didFinishLaunching的地方追加如下代码：

-(void)dealHeader{
    SDWebImageDownloader *imgDownloader = SDWebImageManager.sharedManager.imageDownloader;
    imgDownloader.headersFilter  = ^NSDictionary *(NSURL *url, NSDictionary *headers) {
        NSFileManager *fm = [[NSFileManager alloc] init];
        NSString *imgKey = [SDWebImageManager.sharedManager cacheKeyForURL:url];
        NSString *imgPath = [SDWebImageManager.sharedManager.imageCache defaultCachePathForKey:imgKey];
        NSDictionary *fileAttr = [fm attributesOfItemAtPath:imgPath error:nil];
        
        NSMutableDictionary *mutableHeaders = [headers mutableCopy];
        
        NSDate *lastModifiedDate = nil;
        
        if (fileAttr.count > 0) {
            if (fileAttr.count > 0) {
                lastModifiedDate = (NSDate *)fileAttr[NSFileModificationDate];
            }
            
        }
        NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
        formatter.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
        formatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
        formatter.dateFormat = @"EEE, dd MMM yyyy HH:mm:ss z";
        
        NSString *lastModifiedStr = [formatter stringFromDate:lastModifiedDate];
        lastModifiedStr = lastModifiedStr.length > 0 ? lastModifiedStr : @"";
        [mutableHeaders setValue:lastModifiedStr forKey:@"If-Modified-Since"];
        
        return mutableHeaders;
    };
}

然后，加载图片的地方以前怎么写还是怎么写，但别忘了Option是SDWebImageRefreshCached

NSURL *imgURL = [NSURL URLWithString:@"http://handy-img-storage.b0.upaiyun.com/3.jpg"];
[[self imageView] sd_setImageWithURL:imgURL
                    placeholderImage:nil
                             options:SDWebImageRefreshCached];

摘自：http://blog.csdn.net/jeffasd/article/details/53009429

方法二：

在SDWebImageManager.m大约167行的

if (cachedImage && options & SDWebImageRefreshCached) {
         // force progressive off if image already cached but forced refreshing
        downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
         // ignore image read from NSURLCache if image if cached but force refreshing
         downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
 }

中间加上：

 // remove SDWebImageDownloaderUseNSURLCache flag
   downloaderOptions &= ~SDWebImageDownloaderUseNSURLCache;

变成：

    if (cachedImage && options & SDWebImageRefreshCached) {
                // force progressive off if image already cached but forced refreshing
                downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
                // remove SDWebImageDownloaderUseNSURLCache flag
                downloaderOptions &= ~SDWebImageDownloaderUseNSURLCache;
                // ignore image read from NSURLCache if image if cached but force refreshing
                downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
            }

方法三：

[[NSURLCache sharedURLCache]removeAllCachedResponses];
[icon sd_setImageWithURL:[NSURL URLWithString:url] placeholderImage:imageWithName(placeHolder)];

方法4

SDWebImageOptions 有个选项 SDWebImageRefreshCached
个图片缓存了,还是会重新请求.并且缓存侧略依据NSURLCache而不是SDWebImage.URL不变,图片会更新时使用