In the car network, air quality detection is needed. Combined with Modbus protocol, we design an extensible vehicle air quality detection system. In the system, Microprogrammed Control Unit (MCU) STM32F439 is adopted as the main control core and MCU STM32F103 is used as the sensor protocol adapter, which adapts to the gas sensors such as formaldehyde sensor, temperature and humidity sensor, PM2.5/PM10 sensor, unify different sensor protocol into an unified protocol, transmits sensor data to the main control core through RS485 interface on the basis of Modbus protocol. After the main control core filters and processes the data, it will send the data to the acceleration processor unit (APU), and upload the information to the server in real time by means of the GPRS communication module.