A four-layer photothermal radiometry model is proposed and the thermal diffusivity of woven carbon fiber reinforced polymers (CFRP) is researched. The experimental device is calibrated with the glass carbon as the reference material, and the detection tests of the fourteen groups of CFRP samples with porosities of 0 to 18.32% are accomplished. The least-square objective function is constructed, the optimal solution is obtained by the genetic algorithm, and the thermal diffusivities of CFRP with different porosities are acquired. The results show that the thermal diffusivities of the fourteen group of CFRP samples are within the range of 3.01×10-7-8.73×10-7 m2·s-1, which decrease with the increase of porosity. When the porosity is smaller than 1.00%, the thermal diffusivity shows an obvious downtrend, while when the porosity is larger than 1.00%, the descent speed slows down.