For the purpose of measuring physical property of a coating by non-destructive testing (NDT), acoustic characteristics of plasma-sprayed coating has been studied with the ultrasonic testing. The analytic equation of ultrasonic propagation in the coating based on spectrum analysis is deduced. A transfer function H(f) is established, which contains information on the acoustic properties of the coating. Not only the ultrasonic propagation velocity in the coating, but reflection coefficient, attenuation coefficient, density of the coating can be acquired. This method overcomes shortcomings that the acoustic parameters cannot be measured due to mutual superposition of multiple reflected waves at the same interface of the coating. Porosities of the coatings were tested by buoyancy method. It is found these parameters mentioned above can characterize physical property of the coating. The relationships between porosity and reflection coefficient, sound velocity, attenuation coefficient were investigated, respectively.The results show that reflection coefficient and attenuation coefficient increases with the increase of porosity of the coating. The sound velocity decreases with the increase of porosity. The method is suitable for the evaluation of the coating quality.