Abstract: Spatial correlation is an important characteristic of underwater sound field, which is relevant to underwater localization, underwater communication and serves as a useful and efficient tool for predicting the performance of large acoustic arrays. Compared with spatial correlation of sound field excited by a waterborne source which has received lots of attention, there are few researches about the airborne source. Based on the normal-mode theory, a mathematical expression for longitudinal horizontal correlation coefficient of underwater sound field excited by an airborne source is derived in this paper, and a simulation is employed to compare the longitudinal horizontal correlations for source in air and in water. An air-to-water experiment was conducted in the South China Sea, and signals transmitted by a hooter hung on a research ship were received by an underwater horizontal line array during the experiment. It is shown that, for underwater sound field excited by the airborne source at different distances, the longitudinal horizontal correlation coefficient has an evident oscillation structure, which is explained by the proposed mathematical expression.