A nonlinear model based on the diffusion process is established in light of the difficulty of evaluating the life of equipment with traditional performance degradation models. Standard Brownian motion is introduced for the deduction of the first hitting time distribution of crossing fail boundary. Thus, the approximate life time distribution in terms of the first hitting time is obtained. Then, the maximum likelihood estimation method is proposed to estimate the unknown parameters in this model. Finally, the model is used to evaluate and analyze the lifetime and reliability of a set of accelerometers. Comparison with traditional methods proves that the proposed model is more practicable and accessible.