To investigate the lifetime of DC film capacitors subject to humidity environment, this paper proposes a degradation model to extract reliability characteristics from performance degradation measures. The percentage of capacitance loss of DC film capacitors is driven by a non-stationary gamma process and the component heterogeneity is characterized by random effects. Based on the model, the method of maximum likelihood for the model parameter estimation and the reliability characteristics including the probability density function of failure time and its confidence interval are developed. The effectiveness and superiority of the proposed method are illustrated and verified by comparing with the conventional failure time analysis method. The results indicate that the proposed method is capable of obtaining comparable lifetime prediction with reduced accelerated testing time.