Standard impermeability test is used to evaluate the impermeability of concrete without bearing history. For having no test to evaluate fiber concrete with load-supporting history under service environment, a new test, splitting tensile permeability test is proposed in the paper. The proposed test method involves two parts called splitting tensile test and load holding permeability test. The splitting tensile test is used to get fiber concrete specimen with tiny crack which impermeability measured by the load holding permeability test. Based on the concept of water proof grade in underground engineering, a new method for defining fiber content is proposed to improve the impermeability of fiber concrete. The new test and method is used to define the fiber content in a subway station of shanghai metro line 7, which have well impermeability upon engineering completion. So splitting tensile impermeability test is proved to have good reliability and feasibility for evaluating fiber concrete servicing in underground engineering.