You need to ask the new company to file an H1 petition for you (provide a copy of your old company H1 approval notice with the new H1 petition so that USCIS can know you do not need a new h1 quote). If you leave US now, you can ask the company send the h1 approval notice to you after the H1 petition is approved and you can apply for the h1 visa in China.
Anyway, talk to the company and they konw how to do it.
