Re: problem of variable adress in myscellium HD

I created a wallet in myscellium wallet . There was a HD wallet and then I made export the private key .
Now when I made a single transaction the address of wallet changed .

That's what an HD wallet does. It generates a new address right after your previous address received a transaction.
Alcoinhosting is correct. All addresses being generted in that HD wallet belongs to a single privatekey .
See more about HD wallet here: http://support.mycelium.com/hc/en-us/articles/207045475-What-does-HD-in-HD-Wallet-mean-

you mean I can reuse that old address which is no longer is see in the wallet after one transaction

Yes you can still reuse it. It will be received in the HD wallet.