Step 1. Open Help==>Debug Windows==> console tab in Desktop Wallet
Step2 . Just use "dumpwallet /path/to/dumpwallet.txt" in the QT Wallet Console
For Mac/Linux user: Example dumpwallet
For windows user : Because security setting you should dump your wallet to your home folder: dumpwallet "C:/Users/YOUR-USERNAME/dumpwallet.txt"
The command will export every of your private keys into that text file.
P/s: in case you have encrypted Wallet before you need run this command before run dumpwallet
walletpassphrase "your pass phrase" 600
replace "your pass phrase" with real your passphrase that you used to encrypted your wallet