In order to explore the influence law of gaseous water on the mechanical response of asphalt pavement, a three-dimensional finite element model of pavement structure considering gaseous water was established by using finite element software ABAQUS. Based on fatigue life calculation formula and laboratory test, the influence difference between conventional and actual non-uniform water and air concentration fields on pavement fatigue life was analyzed. The results are as follows. The fatigue life calculation of conventional water-gas concentration field is greater than the actual fatigue life of pavement, which overestimates the anti-fatigue ability of pavement structure. Laboratory tests show that the fatigue life of asphalt mixture after the erosion of gaseous water will also shorten its fatigue life, resulting in the lack of safety of pavement. The actual inhomogeneous moisture concentration field can improve the accuracy of the results.