The invention discloses a coating debonding ultrasonic phase spectrum C scanning imaging detection method, and belongs to the technical field of ultrasonic nondestructive detection. The detection method involves an ultrasonic C scanning system composed of a water tank, an ultrasonic water immersion probe, a three-dimensional stepping device, an ultrasonic flaw detector and a computer. Ultrasonic waves of the system are vertically emitted to a coating sample and used for ultrasonic C scanning, aliasing signals Pr(t) of a water/coating substrate interface and a coating/substrate interface are collected, fast Fourier transform is carried out on Pr(t) to obtain an unwrapped phase psi0(f), linear fitting is performed on psi0(f) in an effective frequency band to obtain a phase spectrum psip(f),the amplitude avg_A composed of the maximum value average value and the minimum value average value of psip(f) and a resonant frequency interval df=fm-fm-1 are recognized, a gray value is used for representing normalized avg_A*df which is drawn in a corresponding C scanning sampling matrix, and ultrasonic phase spectrum C scanning imaging of the debonding state of a coating interface can be achieved. The detection method solves the problem that thin coating debonding cannot be detected due to large ultrasonic high-frequency attenuation, low-frequency signal aliasing and the like.