Environmental factors such as temperature, electromagnetic fields, and cosmic rays can affect the reliability of relay protection systems. Thus, for dual-unit protections operating in the same environment, the occurrence of defects is not mutually independent; there exists a phenomenon of defect accumulation. Ignoring the influence of environmental factors and defect aggregation can lead to inaccurate reliability assessment. To improve the accuracy of the reliability assessment for the dual-unit, this paper proposes a reliability assessment model for dualized relay protection systems considering environmental factor classification (EFC) and defect aggregation phenomena (DAP). In detail, first, the problems with traditional models in the reliability assessment of dualized relay protection systems are analyzed. Secondly, considering the phenomenon of defect accumulation in dualized relay protection systems, an eight-state Markov model that considering the EFC is established, and the probability of defects occurring in the dualized relay protection system under this model is derived. Finally, the accuracy of the proposed model is verified through model comparison.