Grating lenses are diffractive optical elements with gradually variant space and line, and are widely applied to various optical systems, such as harmonic wave separation and diagnosis of high power laser system, optical communication and spectral analysis. Because of its small feature size (just about several times of the illumination wavelength) and gradually variant space and line, the simulation results are not accurate when using the scalar diffractive theory. In this paper, the grating lenses are subdivided into smaller areas, and every sub-area is regarded as periodic microstructure because variance of adjacent period is very small. Then Fourier modal method is adopted to analyze their diffractive properties in sequence, and finally total diffractive efficiency of grating lenses can be easily obtained. Its physical concept is clear and concise, and computation cost is small. Through numerical simulation, diffractive efficiency of grating lens for beam sampling and harmonic separation in high power laser system is calculated. It indicates that the method presented in this paper is accurate and valid. In addition, fabrication errors effects on diffractive efficiency are also simulated in order to obtain the relationship between process errors and diffractive efficiency of grating lenses. It suggests that grating lenses not only can be easily realized in fabrication process, but also can meet practical demand in high power laser system. 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.