Abstract: An improved one-dimensional optimization algorithm was proposed in order to reduce the workload for calculating the non-probabilistic reliability index. After an interval variable is transformed into a normalized one, the limit state equation referred to as the failure surface is obtained. The monotonicity of the objective function has been simply determined by narrowing the scope of interval variables in normalized infinite space. Moreover, the calculation steps of the improved semi-analytic approach were presented to display the solving procedure of the non-probabilistic reliability index. Non-probabilistic reliability evaluation about bridge in service was discussed. The effectiveness and practicality of the improved one-dimensional optimization algorithm has been proved to be suitable for solving non-probabilistic reliability index of bridge in service.