那你使用一下UIImagePNGRepresentation

Instead, create a small buffer, repeatedly filling this buffer with portions of the original asset via getBytes, writing this small buffer to a temporary file using NSOutputStream as you go along.

static NSInteger kBufferSize = 1024 * 10;

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
    NSURL *url = info[UIImagePickerControllerReferenceURL];

    [self.library assetForURL:url resultBlock:^(ALAsset *asset) {
        ALAssetRepresentation *representation = [asset defaultRepresentation];
        long long remaining = representation.size;
        NSString *filename  = representation.filename;

        NSString *documentsPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
        NSString *path = [documentsPath stringByAppendingPathComponent:filename];
        NSString *tempPath = [self pathForTemporaryFileWithPrefix:@"ALAssetDownload"];

        NSOutputStream *outputStream = [NSOutputStream outputStreamToFileAtPath:tempPath append:NO];
        NSAssert(outputStream, @"Unable to create output stream");

        [outputStream open];

        long long representationOffset = 0ll;
        NSError *error;

        uint8_t buffer[kBufferSize];

        while (remaining > 0ll) {
            NSInteger bytesRetrieved = [representation getBytes:buffer fromOffset:representationOffset length:sizeof(buffer) error:&error];
            if (bytesRetrieved < 0) {
                NSLog(@"failed getBytes: %@", error);
                [outputStream close];
                [[NSFileManager defaultManager] removeItemAtPath:tempPath error:nil];
                return;
            } else {
                remaining -= bytesRetrieved;
                representationOffset += bytesRetrieved;
                [outputStream write:buffer maxLength:bytesRetrieved];
            }
        }

        [outputStream close];

        if (![[NSFileManager defaultManager] moveItemAtPath:tempPath toPath:path error:&error]) {
            NSLog(@"Unable to move file: %@", error);
        }

    } failureBlock:^(NSError *error) {
        NSLog(@"assetForURL error = %@", error);
    }];
}

- (NSString *)pathForTemporaryFileWithPrefix:(NSString *)prefix
{
    NSString    *uuidString = [[NSUUID UUID] UUIDString];

    // If supporting iOS versions prior to 6.0, you can use:
    //
    // CFUUIDRef uuid = CFUUIDCreate(NULL);
    // assert(uuid != NULL);
    // NSString *uuidString = CFBridgingRelease(CFUUIDCreateString(NULL, uuid));
    // CFRelease(uuid);

    return [NSTemporaryDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:@"%@-%@", prefix, uuidString]];
}