This paper systematically studies the mechanism of evaporation CO2 laser cutting high absorptive nonmetallic material. Based on the cutting front and kerf shape photographed in the experiment, a three-dimensional energy coupling model for evaporative laser cutting nonmetallic material is established through theoretical analysis and the reflective transmission and energy absorption of laser against real bended cutting front and both two walls are analyzed. The front and wall's energy absorption is mainly determined by the first three incident beams, that is two reflections. The multiple reflections from the front will increase the power intensity of the bottom and the multiple reflections from the wall will increase the power intensity from the center to the bottom. Due to the multiple reflections, the power intensity distributes over the total front. Both two walls will have waveguide effects on the incident light which will mutual reflect against both walls and transmit toward the bottom of the cut, that is so-called “wall focusing effect”.