Abstract: It is usually assumed that the strength of individual particles deteriorates with time when using the discrete element method to model the creep behaviour of rockfill materials under load. Quantifying the time-varying degradation characteristics of the actual particle strength is essential for a quantitative analysis. However, effective means to measure these characteristics are currently lacking. To address this issue, a measurement method that integrates experimental testing with theoretical analysis is proposed in this paper. In this method, the creep compliance of individual particles is determined by a uniaxial compression test with a multiple-stage constant loading history. Moreover, theoretical analysis is employed to obtain the spatiotemporal evolution of the strain field in individual particles under a single-stage constant loading history. The calculation equation for the particle failure time is then derived based on the maximum tensile strain theory. Finally, the time-varying degradation process of the particle strength is characterized by a quantitative relationship between the load level and the failure time. Preliminary applications on limestone particles (10 − 50 mm) demonstrate the effectiveness of this method. The results also exhibit that the relationship between the load level and the failure time follows an exponential function. Meanwhile, the decay parameter shows a lognormal distributional randomness, and the randomness of the load level corresponding to the long-term strength obeys a beta distribution. The proposed method provides a viable solution to quantitatively measure the time-varying degradation characteristics of rockfill particle strength.