The influence of plume on transmittivity during laser irradiating c-epoxy composites in gas flow is researched. A calculated model for transmittivity of plume is given by using Lambert- Beer theory. In the model, the length of optical paths is calculated by jet theory, density and velocity of plume can be got from the simulation of laser ablating carbon/epoxy composites. The plume transmittivity during laser irradiating is computed by the model. The result is compared with the experiment, and it is confirmed that the model is reasonable. The influence of extrinsic parameters on laser transmittivity is calculated. The results indicate that when laser irradiation begins, thermal decomposition zone is on the material surface and it is easy for plume escaping from the zone, and transmittivity decreases to minimum value quickly. Then thermal decomposition zone moves to the interior and it is difficult for plume escaping from the zone because of low permeability, and transmittivity increases slowly. Higher laser intense decreases the minimum transmittivity and average transmittivity of the whole irradiation time; higher flow velocity can increase the transmittivity of the whole irradiation time; when total laser power is constant, bigger spot size can increase the minimum transmittivity and decrease average transmittivity of the whole irradiation time.