Research conclusions :(1) The objective reliability index is dominated by the smallest one of the whole possible failure modes. Partial coefficient can be calculated based on a given target reliability index. (2) Reliability index values are different when computed by different methods. Based on calculation methods and calculation example, contribution value method is considered more accurate, handy and practical when compared with the result from the other computational methods. Contribution value method is recommended as the best calculation way for the target reliability index for tunnel portal structure. (3) The processes of contribution value method for partial coefficient are described, and the mathematical description of the method is given clearly. Contribution value method is proved to be feasible through the calculation example. (4) Among five partial coefficients, the greatest impact to the limit state of tunnel portal structure is the partial coefficients of coefficient of earth pressure. So the study of statistical character for soil pressure should be enhanced. (5) The method for determining target reliability and partial coefficient of railway tunnel portal can directly be applied to the design of railway tunnel portal structure.