Currently I own a lot of altcoin, I bought them at ico. My account is being downgraded several times. There are many people who say bitcoin will continue to decrease, so altcoin prices also decreased compared to USDT. So now, should I sell them and wait for the price to buy back? Give me advice 
If you sell them now, you're gonna go down, and you'll never have a chance to get your money back. Better wait. All your coins can't go down. Then you're just an unlucky guy who shouldn't be in this business.