In the measurement of sound velocity in water，abnormal phenomenon appears that the magnitude of locally maximum of sound pressure on the receiving transducer increases as the receiver moves away from the emission transducer，which differs from the prediction of ideal model and conflicts with the common sense of attenuation in water.Furthermore，the relationship of the locally maximum pressure and the distance between two transducers depends on the height of water in the tank.To solve the myth above，the superposition of Gaussian beams and the method of image sources are applied.Equipped with the analytic expression of the relationship between the pressure and the distance，numerical simulation reproduces the non-monotonic decreasing as seen in the experiment，thus the speculation that the reflection on the interface of water-air and water-tank contributed to such phenomena is verified.