i find when i use a new chrome browser in an new computer,
i download and install metaMask,when i input my password and my Mnemonic Phrase，then when i create new eth address.
Miraculous things come up.my new eth address is what I had created before.
what happened？
my private key is remembered by the metaMask？
No, your private key is not remembered by the metamask extension. You don't need a private key if you use a mnemonic phrase. Mnemonic recovery phrase or mnemonic seed is a list of words which store all the information needed to recover your wallet.
In your case, dddonnano, it is about lack of knowledge rather than the security issues of the Metamask Extension. I suggest that you learn more about the basic concepts and technologies of the crypto world.