People seldom mix thermal sensitive polymer together with another kind of thermal resistant polymer to avoid thermal degradation during the processing. This selective elusion is mainly base on a hypothesis that degradation of one component will absolutely lead to certain deterioration in blend’s mechanical properties. While, an interesting question here is that does the hypothesis be true at any cases? According to the Doctrine of Golden Mean, an important principle of Confucius and Aristotle's ethics, we always can find a golden mean way between two very different methods. Therefore, we might find an actual golden point between blend and thermal degradation, control and utilize the degradation process of the thermal sensitive component by adjusting the processing parameters. In this paper, we demonstrate that it is possible to fabricate a polymer blend to verify the above assumptions, polyphenylene sulfide (PPS) and polymethyl methacrylate (PMMA) were blended above 300oC (about the thermal degradation temperature of PMMA) in this work. Mechanical and thermal properties of the blend and the thermal degradation process of PMMA were investigated. It was shown that during the extrusion process PMMA underwent a procedure of thermal-degradation which led to cavitations in the matrix, concentration of internal stress and alternation of mechanical properties of the blends. Comprehensive analysis of the experimental results demonstrated that the in-situ thermal-degradation process of PMMA do can give positive effect on mechanical properties of the blends, and the degree of PMMA thermal-degradation can be used as an additional parameter by which to tune the characters of the PPS/PMMA blends. The mechanism of the material fracture toughness enhancement caused by in-situ thermodegradation of PMMA was also proposed. Keywords: Polyphenylene sulfide, In-situ Thermal-degradation, Blend, Mechanical properties, Thermal properties