Abstract: The thermal blockage effect caused by plasma (produced by mega-watt laser ablation of C/C composite target) absorption laser beam energy is studied in this paper. First, based on the inverse bremsstrahlung absorption theory, a propagation model of laser through plasma generated by ablating the target is built. Then, based on the theory of magnetic fluid, wave equations of plasma in the form of electromagnetic caused by mega-watt laser and thermal blockage effect model caused by the plasma absorption laser energy are introduced. Finally, numerical simulation gives the total density of particles in the ablation process, absorption coefficient, changes of target surface equivalent heat flux and whether thermal blockage effect is taken into account, the differences of the temperature field curve along the vertical direction of the target surface. The results show that because of the plasma formation, the thermal blockage effect is obvious to the laser. It weakens the burning effect of laser on the target, and makes the total particle density, the absorption coefficient, the equivalent heat flow on target surface and temperature field vertical to target surface all change nonlinearly.