Abstract: In order to learn the air-to-water sound transmission characteristic in shallow water, an experiment was conducted in South China Sea in July 2010. During the experiment, sound pulses sent by a loudspeaker hung on a ship were received by an underwater HLA. The distances between the loudspeaker and the HLA were from 1 km to 4 km. The pulse compression and conventional beam forming are used to increase the signal-noise-ratio. And the air-to-water sound wave transmission loss at the frequencies from 200 Hz to 1000 Hz is estimated. The transmission loss calculated by a wave number integration model is also provided, which is in well agreement with the experimental data.