A reliability assessment model based on the total probability formula is proposed to address the potential fatigue failure and fracture failure of MEMS acceleration sensors under complex stress conditions. The model accomplishes reliability modeling of the device in vibration, impact, and vibration-impact coupled environments. The model includes the Wiener processes and the homogeneous poisson random processes, which describe the fatigue damage of the device in the vibration environment and the random impact of the device, respectively. Furthermore, the influence of the amplitude of random impacts on the device degradation rate is considered. The correlation between multiple failure modes is reflected by the sudden increase in fatigue damage generated by the device under impact stress. A comparative analysis was conducted to compare the reliability models considering the independent and coupled effects of vibration and impact. The results demonstrate that the reliability model considering the coupled effects of vibration and impact provides more meaningful guidance in the evaluation.