Abstract: An inversion method based on ultrasonic reflection coefficient phase spectrum (URCPS) combined with acoustic impedance constraint was proposed to simultaneously measure density and longitudinal wave velocity of thin coatings. Ultrasonic pulse echo method was employed on aluminum alloy plate with thickness of 1.929 mm and epoxy resin coatings containing 30%-50% iron powder with thickness between 0.2-0.5 mm to obtain ultrasonic signals. Then an inverse algorithm based on the correlation coefficient method was introduced to retrieve the density and longitudinal wave velocity by comparing the theoretical and experimental URCPS and searching maximum in correlation coefficient matrix constrained by acoustic impedance measured by ultrasonic method. The relative errors of the retrieved density and longitudinal wave velocity were less than ±3.2%.