This paper introduced reliability theory to the calculation of airport runway length. The random variables relevant to the runway length were analyzed, and distributive models were gained for relative air density, wind velocity, pilot error coefficient and distance from landing site to runway end. With genetic algorithm and importance sampling, a new design method of runway length was put forward based on reliability theory. The reliability of long-term airport was calculated with this method, the calculating failure probability was 4.75×10
-4, and practice result was 2×10
-4~3×10
-4, two results were closed. The results indicate that the method is feasible in the design of runway length and safety evaluation.