A new method is proposed to measure the photon orbital angular momentum (OAM) carried by helical beams accurately through the amplitude computer-generated hologram (CGH) loaded on a liquid crystal display spatial light modulator (LCD- SLM).The diffraction processes of Hermite- Gaussian beams,Laguerre-Gaussian beams and Bessel beams with different orders passing through diffractive optics such as circular aperture,triangular aperture,Young’s double- slit,grating whose parameters are adjustable,are simulated with the help of plane wave angular spectrum diffraction formula and Collins formula.Then the propagation properties of helical beams passing through Young’s double- slit and triangular aperture are studied by theoretical simulation and experiment,and the measurement of photon OAM carried by helical beams is also realized with those two amplitude diffractive optical elements.Due to the fact that changing and controlling the CGH loaded into the SLM is able to change the configuration,dimension and location of diffractive optical elements flexibly and accurately,the accurate measurement can be achieved conveniently.