Visa for cambodian..

Hey, the single-entry type allows travelers to enter mainland China once, while the double-entry type allows them to enter twice. In your case, a single-entry type will be enough for your travel, so the cost will be 20 dollars.

thanks.. But how can I apply? Do I need to go to Chinese embassy?

You should first get all the required documents ready, such as your passport, a photocopy of the passport's info page, a application form stuck with your passport type photo, round-trip tickets and hotel reservations, and other materials requested by the embassy. Then go to the embassy to submit the application.