Abstract: A Certain quality of polymer melt in sealed container, the PVT state variables conform to PVT state equation of the ideal gas. Use this the characteristics of polymer melt, By the experimental device based on PVT principle to design processing, the mathematical expression of experiments on polymer melts density and polymer bulk elastic modulus on the basis of physical definitions can be inferred. Property verification consistent changes exist in theory of polymer melt density and polymer bulk elastic modulus. Meanwhile, from the interrelationship among polymer melts density, polymer bulk elastic modulus and ultrasonic propagation velocity, the single valued mathematical expression between ultrasonic velocity and polymer melts density can be deduced and one-to-one correspondence between polymer melt density and ultrasonic velocity can be verified by experiments. This provides the theoretical basis for the application of ultrasonic technology to online measurement of polymer melts density.