Myanmar citizen can apply the double entry China visa in Phnom Penh China embassy?

You'd beeter apply for a China visa from the Embassy or consulate of China in your own country in advance, especially that you are planning to apply for a multiple entry China visa. As I know, most China embassies and consulates don't issue China visa to temporary visitors.
To apply for a multiple entry China visa, you must have been to China in the recent two years and you have to provide acceptable proof of frequent travel to China.