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to use visa waiver at first is realy like a 餿主意 :), for it wont work. once you have a chinese visa, they will activate the visa when you enter china. you can not tell them to use your visa-waiver for the first 15 days.

solution:
1. before 60 days is over, go out and re-enter.
2. try to extend your stay in china by visiting the entry-exit bureau. However, if you converted to singapore passport from a chinese passport holder, they will ask you to concel your HU KOU in china first...