If I have valid USA entry permit, then is it possible to apply VOA or advance visa is needed?

Patel, having a valid USA entry permit will not enable you to apply for the VOA. However, in certain cities, such as Shenzhen, Zhuhai, Xiamen, Shantou, and Hainan, it's possible for you to get the VOA. If you want to travel in other parts of mainland China, you have apply for one entry permit in advance.