The distribution of optical field passing through lens array is analyzed. Lens array consists of many array units, which can improve energy distribution uniformity around the focus of main lens. Lens array improves illumination, but it brings bad affection to energy measurement. The reason is that intervention will happen between sub-beams after the array units, thus energy density and power density of laser on the interference stripes will increase sharply. Its value will augments to four times contrasting to former one. The higher energy and power need more severe criteria to calorimeter. If proper means is not taken in use, calorimeter will be destroyed.