I didn't use it for more than half an year. When I took it out, I saw the inside of the pot is oxidation or rust, is it safe to use it? How can I clean it?
This looks like oxidation, try cleaning it with hot water (NO soap). After most of it has gone, fill up with hot water again and add a few spoons of bicarbonate. Stir and let it sit for a day or 2. After that, rinse and inspect. If the oxidation has gone too deep, the pot will need sanding and polishing, which will weaken the pot. In that case it would be better to ditch it and get a new one.