The random coefficient regression model is a very important model for describing the degradation process. This paper studies the degradation process based on the coefficient regression model, and the covariates are added in the process according to the accelerated failure time model. Two renewal thresholds are studied, one is preventive maintenance renewal threshold, and the other is failure renewal threshold. When the degradation process reaches or exceeds the preventive maintenance renewal threshold, the system should be renewed by preventive maintenance, and then a preventive maintenance renewal cycle is finished; when the degradation process reaches failure renewal threshold, the system should be renewed by replacement, and then a failure renewal cycle is finished. This paper develops the model of the expected cost per unit time in the renewal cycle using the renewal reword theory. A simulation algorithm is developed to verify the cost model. Numerical examples are studied to demonstrate the model and the simulation algorithm.