As a core packaging material of light color conversion, phosphor/silicone composite plays an indispensable role in light emitting diode (LED) packaging. At present, commercial LED packages mainly use blue LED chips to stimulate Yttrium Aluminum Garnet (YAG) yellow phosphor to reach a white color. However, (Ca,Sr)AlSiN 3 :Eu 2+ (CSASN) red phosphor is often added to improve the color-rendering performance given the absence of red light emission spectrum. However, inevitably harsh working conditions can induce the degradation of CSASN red phosphor, which will directly influence the mechanical properties of its silicone composites and challenge the reliability of its LED packaging. In this study, the coupling effects of temperature–humidity–sulfur on the mechanical degradation of CSASN phosphor/silicone composites were considered. The prepared CSASN phosphor/silicone test samples were first aged under high-temperature, high-humidity, and high-sulfur conditions. A series of dynamic mechanical analysis tests were then conducted to qualitatively evaluate their mechanical properties. Finally, the dynamic tension process and interfacial cracking of CSASN phosphor/silicone composites were simulated by using finite element analysis with cohesive modeling. The results showed that: (1) under coupled aging conditions, the mechanical properties of the phosphor/silicone composite decreased due to the reaction of phosphor with sulfur, water, and oxygen; (2) crack initiation and propagation were most likely to occur at the edge of the crack perpendicular to the tensile direction. The debonding of particles with silicone rather than the fracture of phosphors was one of the main aspects resulting in failure mechanisms; (3) the highly concentrated and localized phosphor in the silicone matrix and the irregular shape and arrangement of phosphor particles generated cracks in the phosphor/silicone composite.@en