Evaporative cutting front shape of PMMA by CO2 lasers is photographed through CCD camera. Effects on cutting front shape and depth by cutting parameters as defocus distance, cutting speed and laser power are discussed. An energy coupling model is built up on the front and kerf shape photographed. The results indicate that multiple reflections are favorable to the increase of depth. When the focal position is above the surface, the absorptive intensity on the front decreases, so the cutting depth decreases. When the focal position is below the surface, the position on the front with the maximum power intensity is downward. The intensity distribution is toward the deeper position, the cutting depth increases. But if the negative focal distance is extremely big, the intensity of the deeper position attenuates sharply with the increase of focal radius and light distances, so the depth deceases. With reducing the cutting speed and increasing the laser power, the curvature decreases, the length of front directly irradiated increases, and thus the cutting depth increases.