Onshore wind turbine foundations subjected to the relentless cycle of fatigue loads, are prone to fatigue-induced failure. Consequently, the accurate assessment of the fatigue reliability of these foundations is of significant importance. This study initiates with an in-depth exploration of the propagation of randomness within physical systems, elucidating the fundamental principles of probability density evolution theory. Building upon this theoretical foundation, a probabilistic evolution equation tailored for wind turbine foundations under stochastic fatigue loads is formulated, thereby advancing an innovative approach to the fatigue reliability analysis of such structures. Finally, the case study of a wind turbine foundation located in Ruyuan County, China, is presented. The fatigue reliability of the wind turbine foundation throughout its service life is evaluated. The study’s findings demonstrate that the developed methodology is capable of effectively evaluating the fatigue reliability of wind turbine foundations throughout their entire life cycle, which holds significant scientific implications for the fatigue-resistant design and safety evaluation of turbine foundations.