RTV (Room Temperature Vulcanizing silicone rubber) is widely applied to prevent serious pollution flashover accidents on the porcelain and glass insulators. The substandard coating thickness will seriously affect the insulation performance of the insulator and bring hidden trouble to the safe and reliable operation of the equipment. This paper proposes a nondestructive detection technique for RTV coating quality based on photothermal radiation measurement. Firstly, using COMSOL simulation methods, RTV coating models of insulators of different thicknesses are created to solve temperature curves under pulse excitation. Fast Fourier Transform (FFT) transformation is performed on the descending part of the temperature curve. In the spectrum diagram, the coating thickness is characterized by two different thermal properties in the frequency domain, the coating thickness is fitted to the phase at a specific frequency and the reciprocal of the square root of the minimum phase frequency. Both methods can characterize the coating thickness and the characterization effects are validated through experiments. The research shows that using the thermal characteristics of this frequency domain can achieve quantitative detection of insulator RTV coating thickness, which will become a portable, safe, large-scale, non-destructive, on-site RTV coating thickness detection method.