This paper proposes an automatic method to analyze the reliability of the protection system in smart substation utilizing the Substation Configuration Description (SCD) file. First, the predicate expressions for virtual terminals and virtual circuits are derived from the SCD file, and the graph of logical connections of the protection system is constructed accordingly. Then, based on the modeling of Ethernet switch and optical fiber, the physical connections are recognized through predicate calculus, and the graph of physical connections is then constructed. Last, the graph of physical connections is transformed to reliability block diagram, thus the reliability calculations can be carried out. A typical 110 kV power transformer protection system is used as an example to demonstrate the effectiveness of the proposed method.