In this study, we analyzed the degradation behavior of fluoroelastomer (FKM) O-rings and established a correlation between the sealing force and equilibrium compression set, allowing us to predict the residual lifetime of field-aged FKM O-rings to be 12.3 years. An intermittent compressive stress relaxation (CSR) jig was designed based on the working mechanism of O-rings, enabling us to measure the sealing force of an FKM O-ring through intermittent CSR testing. The measured sealing force was 1.27 kN at room temperature. We observed the degradation behavior of FKM O-rings at various thermal aging temperatures and applied the time-temperature superposition (TTS) principle to obtain the stress relaxation master curve at 60℃. By analyzing the relationship between αT and 1/temperature, we extrapolated αT to be 0.088 at room temperature. We then constructed the stress relaxation master curve at room temperature and defined the failure condition as Ft/F0 = 0.2, which led to a predicted lifetime for the O-ring of 66.2 years at room temperature. The compression set of the O-rings decreased over time at room temperature due to physical relaxation, which induced a recovery of O-ring thickness. We found that thermal treatment could accelerate this physical relaxation, allowing the O-rings to reach an equilibrium state in a shorter time. We recommend a thermal treatment condition of two days at 80℃. To assess the effects of chain scission and chain recombination, we measured stress relaxation in both strained and unstrained states. The average value of parameter a, representing the ratio of chain recombination to chain scission, was found to be 1.47, with a deviation of ±0.05. Because this value was greater than one, we confirmed that chain recombination was relatively dominant in the FKM O-rings throughout the aging process. Using empirical data on the sealing force and equilibrium compression set, we established a correlation between Ft/F0 and the equilibrium compression set. To predict the residual lifetime, we examined 10 FKM O-rings that had been field-aged for 12.3 years in a propulsion unit. Following the recommended procedures, we measured the equilibrium compression set of these field-aged FKM O-rings to be 13.45%, with a deviation of ±4.6%. By analyzing the relationship between Ft/F0 and the equilibrium compression set, we calculated the value of Ft/F0 for the field-aged FKM O-rings to be 0.909. Utilizing the stress relaxation master curve at 23℃, we predicted the residual lifetime of the field-aged O-ring to be 56.3 years.