Ultrasonic phase spectrum of reflection coefficient has been used to nondestructively characterize the density,porosity,and microcracks of plasma sprayed Cr2O3 coatings irradiated by high-intensity pulsed ion beam (HIPIB).The ultrasonic measurement was investigated using immersion focusing pulse echo method with a 25 MHz transducer and the phase spectrum of reflection coefficient has been experimentally obtained.The relationship among velocity,density,attenuation coefficient and the phase spectrum was analyzed based on an acoustic transmission model in a multi-layered structure.The velocity relates to the harmonic frequency of the phase spectrum,and the density and attenuation coefficient play roles on the amplitude of the maximum and the shape of the spectrum.