The shear viscosity of concentrated aqueous solution of polyacrylamide is so high that, according to the classical theory of the aioustic absorption, ultrasonic vibration can not propagate in it at all. However, our experiment with ultrasonic wave of frequency 11.4 MHz finds that it exhibits ultrasonic absorption with the same order of magnitude as water does. This paper gives a qualitative explanation about the great difference between theory and experiment on the ground of molecular structural feature of this kind of solution.The measurements of the ultrasonic absorption of acrylamide aqueous solution with various concentrations have also been carried out. By comparing the experimental results of the two solutions it can be considered that the excessive ultrasonic absorption of the polyacylamide solution comes from the characteristic behavious of the solute macromolecules themselves.The ultrasonic velocity and density of these two solutions are also investigated. From them the concentration curves of compressibilities has been calculated and it is found that some atomic groups in the solute molecules exhibit the same hydrate behavior in the aqueous solution independent of whether they are in polymer or in small molecular substance.