With the improvement of voltage level of electrical equipment, the requirement for the reliability of polymer dielectric materials in electrical equipment is much more strict. However, there are inevitably some defects in the manufacturing process of polymer dielectric materials, such as bubbles, inclusions, and so on. Therefore the life of electrical equipment will be greatly decreased when partial discharge occurs in a high electric field. In order to prevent and reduce the accident caused by the aging of polymer dielectric material in the electrical equipment, electrical aging life needs to be reasonably estimated. Based on the microscopic mechanism of partial discharge aging model, this paper suggests a stochastic differential equation for the electrical crack growth by using a non-equilibrium statistical physics theory, and then functions of breakdown probability, reliability, and electrical aging life due to the application of electric field are derived. Finally, a detailed analysis for the polyetllyleneterephthalate (PET) film is carried out, and the theoretical life of electrical aging is compared with experimental data. Results show that the theoretical values are consistent with the experimental data. So the aging life equations of polymer dielectrics derived in this paper can be used effectively in the quantitative analysis and may be helpful for the estimation of electrical life.