Grating lenses have been widely applied to various optical system because of their special optical characteristics. Theoretically, the grating lens is subdivided into smaller sub-areas in this paper, and every sub-area is regarded as periodic micro-structure, according to lightly variance of adjacent linewidth. Rigorous modal theory is used for analyzing their diffractive properties in sequence, and finally total diffractive efficiency of grating lenses can be easily obtained. Numerical simulation is performed to calculate diffractive efficiency of grating lens, which is used for beam sampling and harmonic separation in high energy laser system. It shows that results computed by the method in this paper are accurate and valid. In addition, fabrication-process-tolerance effects on diffractive efficiency are also discussed. In experiment, a beam sampling grating with diameter 100 mm was fabricated, and its experimental diffractive efficiency is consistent with result calculated by the method in this paper.