I used to be a citizen of China, but later I immigrated to Singapore and want to apply for citizenship of China. What should I do?

That will be very difficult. Look at China. In the past few decades, only a few people have obtained permanent resident status, let alone citizens. But if you have relatives in China, you should be able to apply for permanent residence in China. It is possible in policy. Domestic affairs and policies are one thing, but practice is another.

If you can't make up your mind, don't become a Singapore citizen. If you change, don't even think about getting China citizens back. You can't have your cake and eat it. Singapore's policy towards permanent residents is getting worse and worse, but it seems to be quite good for citizens, so there is no need to change it back.