Aiming at the problems of low accuracy of traditional ultrasonic thickness measurement methods caused by thin resin-rich layer on the surface of carbon fiber reinforced polymer (CFRP), similar acoustic parameters between the coating and the matrix, and multiple interfaces, a new method based on ultrasonic echo phase derivative spectrum (UEPDS) is proposed to measure the thickness of resin-rich layer on CFRP surface. Based on the multiple reflection rules of ultrasonic waves in multi-layer structures, the UEPDS of coupled medium/coating/substrate three-medium two-interface structure is constructed, and the theoretical relationship between the resonant frequency of UEPDS and coating thickness is identified, so as to overcome the problem that the accuracy of thickness measurement by the traditional sound pressure reflection coefficient amplitude spectrum or phase spectrum is greatly affected by the reference wave and the initial phase of the signal. Combined with calibrated sound velocity, the coating thickness can be accurately determined. The thickness of the resin-rich layer varying from tens to hundreds of microns was measured by an ultrasonic C–scanning system combined with a nominal frequency 25 MHz probe, and the thickness was compared with that observed by laser confocal microscopy. The results show that the 25 MHz probe can effectively measure the thickness of the rich resin layer with a thickness ≥40.7 μm, and the absolute error between the ultrasonic thickness cloud image and the metallographic observation thickness ≤5.1 μm, and the relative error <8.0%. The research shows that the main factor affecting the accuracy of thickness measurement is the presence of fluctuating braided gap residual resin at the resin-rich layer/matrix interface.