Research conclusions :(1) According to the failure models of railway tunnel portal structure，it is considered to be in series system. Reliability index and failure probability of structural systems are employed in quantitatively evaluating reliability of railway tunnel portal structure. An approximate method is a kind of simple and accurate computing way to calculate the reliability of railway tunnel portal structure. (2) An approach for reliability calculation method of railway tunnel portal structure is given，and demonstrated by examples. A calculation example shows the safe probability of end wall portal structure system of single track railway tunnel that allows the speed of 140 kir^h can be more than 99. 22%， and the main failure mode of structure system is overturning. (3) In this paper， the tunnel portal limit state design method can be directly used to guide the design of railway tunnel portal structure.