您应该直接将文件下载到文档目录的文件夹中，为此您需要创建目录为@iPhone Developer建议并在该目录中写入文件

You should directly download file to folder of document directory and for that u need to create directory as @iPhone Developer suggested and write file in that directory

例如：

  NSString *stringPath = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)objectAtIndex:0]stringByAppendingPathComponent:@"/File"];
  error = nil;
  if (![[NSFileManager defaultManager] fileExistsAtPath:stringPath])
    [[NSFileManager defaultManager] createDirectoryAtPath:stringPath withIntermediateDirectories:NO attributes:nil error:&error];

  NSURL *url = [NSURL URLWithString:@"http://www.ndqatar.mobi/18December/admin/rington/133/QatarDay13.mp3"];
  NSData *data = [NSData dataWithContentsOfURL:url];
  if(data)
  {
    stringPath = [stringPath stringByAppendingPathComponent:[url lastPathComponent]];
    [data writeToFile:stringPath atomically:YES];
  }

如果你想要你可以移动这样的文件：

If u want u can move file like this:

 NSString *destPath = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)objectAtIndex:0]stringByAppendingPathComponent:@"/Dest"];
  error = nil;
  if (![[NSFileManager defaultManager] fileExistsAtPath:destPath])
    [[NSFileManager defaultManager] createDirectoryAtPath:destPath withIntermediateDirectories:NO attributes:nil error:&error];
  destPath = [destPath stringByAppendingPathComponent:[stringPath lastPathComponent];];

 [[NSFileManager defaultManager] copyItemAtPath:stringPath toPath:destPath error:&error];