This paper described the present risks existing in subway construction and proposed the idea that reliability theory can be applied in the field of subway construction. Based on this, a parallel-series model of risk control system was established. According to the required target of system reliability, combining with the reliability distribution theory, the reliabilities of each subsystem that constituted the parallel-series model can be determined. Aiming at the basic events that constituted the subsystem, average allocation method was taken to solve the basic events reliabilities and an example was given. This studies showed that reliability theory can be successfully applied in the field of subway construction and can fulfill the optimization of construction safety under a given security target.