Hello, i am Cambodian and im travelling from Phnom Penh to The US but i need a stay permit.

Yes, if you wanna do this, you need to apply for the Stay Permit at the airport.
Complete the "embarkation/disembarkation card", together with your travel documents, boarding pass for the onward connecting flight and travel itinerary, go to the inspection counter of China Immigration Inspection to apply for 24h stay permit. Good luck!