The invention discloses a device and a method for detecting epitaxial wafer temperatures online in real time, and belongs to the technical field of semiconductor detection. The device comprises an MOCVD (metal-organic chemical vapor deposition) reaction chamber, a light source, a beam splitter, a reference light detector, a reflected light detector and a data acquisition unit. According to the method based on the device, thermal radiation attenuation factors and reflectivity attenuation factors, both caused by reaction chamber window coating, for epitaxial wafers can be obtained. By the device and the method, influences of reaction chamber window coating on online real-time temperature detection values can be eliminated, and accuracy of the online real-time temperature detection values can be improved.