The fatigue failure of gear structure in engineering is a process of continuous accumulation of damage. But the traditional analysis method based on static state can't reflect the time-varying characteristics of gear structure with random load. In order to solve this problem, based on the traditional stress-strength interference static model, the dynamic interference model of cumulative damage-critical damage is established, and the corresponding relationship between cumulative damage and reliability of gear structure with random load is obtained. At the same time, the distribution parameters of critical damage are solved by Bayesian theory. So time-varying reliability analysis model of gear structure based on Bayesian theory is established. This method can be used to calculate the time-varying reliability of gear structure conveniently.