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

That depends mate. Check the market, if the coins you are holding would not do you any good right now, and trading it would only lead to losses, then don't do it mate. Be smart with your investments, I am sure that everybody including yourself do business in this market to earn profit, therefore, if the move you will make would just give you losses instead of gains, then why even risk it and go through it. However, if you see that what you will do in terms of disposing your coins would give you profit, no matter how small or big it is, then do as you please. Having small profit is always better than having no profit at all.