Polymer blending is an important method to prepare new polymer materials with high performances without creating new molecules. In most cases, Twin Screw Extruders (TSE) is used for carrying out polymer blending processes due to their unique mixing performances. However, it is a black-box for flow, mixing and reaction performances of polymer blending processes in a TSE due to short residence time and complex operation conditions. For this reason, this work presents a concept of compatibilizer-tracer to take the above challenge. The compatibilizer-tracer bears reactive groups capable of reacting with its counterpart upon forming a copolymer for in-situ compatibilization of immscible polymer blend, and simultaneously bears fluorescent labels which allow determining its concentration as low as of the order of ppm. The use of the compatibilizer-tracer together with in-line devices has allowed assessing the effects of processing parameters mixing performances of a TSE such as screw configuration, blend composition and screw speed. The results provide fundamental insights into flow, mixing and reaction performances of a TSE for polymer blending. The authors thank supported by the National Key Research and Development Program of China (2016YFB0302202).