The Cox’s time-dependent proportional hazard model (PHM) is widely used. Its hazard function contains two parts: the baseline hazard function which represents the aging process, and the link function which represents the equipment status. Based on PHM this paper proposes a new failure rate model for oil-immersed transformer concerning both aging process and equipment inspection information. The baseline hazard function follows a temperature-based aging model. The quantity of the dissolved gas is used as the covariate of the link function, for its comprehensiveness. We determine the probability density function of the time to failure, and the maximum likelihood estimation (MLE) is used to estimate the model’s parameters. The case study proves the efficiency of the proposed model, which is able to characterize the failure rate of oil-immersed transformer reasonably and comprehensively and can represent the maintenance effect properly.