A processing chamber gas detection system is provided, including a chamber, an exhaust pipe, a connection pipe, and a gas detector. The chamber is configured to perform a chemical vapor deposition (CVD) process. The exhaust pipe is connected to the chamber and the pumping unit, and the connecting pipe communicates with the exhaust pipe. The gas detector is disposed on the connecting pipe and configured to detect the oxygen content in the air from the chamber. When the air in the chamber is pumped out via the pumping unit and the air flows through the exhaust pipe and the connecting pipe, the gas detector detects whether oxygen is contained in the air or not.