In the case of infrared thermography of semitransparent materials, the heat stimulation mechanism differs from that of opaque materials, and the heat absorption of a specimen depends on its optical properties and the radiation spectrum of a light source used. Based on the spectrum characteristics of semitransparent materials, a body heating mechanism and modelling were proposed. To obtain the spectrum absorptivity of the materials, the reflectivity and transmittance of the glass fiber reinforced plastics with different thicknesses were measured in a certain wavelength range. The spectrum characteristics of heating lamps were described by a color temperature model, the effect of the color temperature of a flash on the flaw detection was analyzed by using finite element method, and the relationships between the detectable information parameters(the maximum temperature difference and maximum contrast) and the flash color temperature were presented. The results show that the maximum temperature difference and maximum contrast are a nonlinear function of the color temperature, and they decrease first and then increase when the color temperature increases, so a low or high color temperature is more favorable for the inspection of semitransparent composite materials. The conclusions provide a theoretical reference for the flash infrared thermography of semitransparent composite materials.