An in-car air pollution prevention system is disclosed and includes plural gas detection modules and a gas conditioning device. The in-car gas detection modules and the out-car gas detection modules are provided for detecting the external gas and the air pollution source and transmitting the gas detection data respectively. A plurality of filtering and purification components are disposed within the gas conditioning device for filtering the external gas and the air pollution source. After the control drive unit compares the gas detection data, the gas conditioning device intelligently select and control the introduction or not introduction of the external gas. The plurality of in-car gas detection modules control the enablement of the gas guider of the gas conditioning device under a monitoring mechanism state. Whereby the gas pollution source in the interior space of the car is filtered through the filtering and purification component, and the gas pollution source is filtered and/or exchanged to generate clean air.