This study established a multiscale framework linking SiR (silicone rubber) thermo-oxidative aging to interfacial ultrasonic signatures in cable joints. By setting the aging test at different stages of 0–70 days at 125 °C, the results showed that, the density of SiR elevated by 34.7% and the elastic modulus enhanced by 39.8% driven by cross-linking reactions, while the SiR/XLPE interfacial pressure declined by 18.6% due to stress relaxation. Implementation of these parameters into a sequentially coupled stress-acoustic COMSOL model simulated wave propagation dynamics, revealing that the reflection coefficient of the primary echo increased monotonically with the extension of aging time, which is attributed to the decrease of the interface contact area. And the nonlinear coefficient β exhibited piecewise variation—initially decaying rapidly (harmonic attenuation predominated) and then stabilizing at small interfacial pressures. These findings establish ultrasonic nonlinearity as a sensitive indicator of interfacial pressure deterioration, enabling non-destructive assessment of interfacial integrity in 35 kV–110 kV cable joints.