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 
You should always remember the reason why you invest on that altcoin before taking any steps. To sell or to hold is a personal work which is needed after personal research.