i copy wallet.dat into roaming folder ,but at start when i start litecoin client it overwrites my wallet.dat with a new wallet.dat
Strange one. Is the old wallet.dat from a node of the same version? Is it from a different OS - windows hides extensions sometimes - have you double checked the wallet.dat file properties? Finally - are you sure it's a Litecoin wallet.dat - most Bitcoin fork/clones use the same filename and default directory structure.