Flash infrared thermography (IRT) is a non-contact optical non-destructive technique, which has a promising application in the on-line inspection of disbonds in insulator of solid rocket motors (SRMs). In order to promote the application of IRT in the disbond inspection of insulators in SRMs, the basic regularities of pulsed thermography (PT) of disbonds in the insulator of SRM cases are studied by using numerical simulation and experiment. The relationships between decision-making parameters and structure parameters are obtained, the criterion to determine detection limits is proposed, the detection limits of disbond size and depth (i.e. insulator thickness) are calculated, and the method to characterize the disbond size and the error correction formula are provided. The equations, data and conclusions provide a scientific basis and operational guidelines for the infrared thermography of disbonds in insulator of SRMs.