For traditional conic refractive laser beam expanders being difficult of fabrication, hard of equipment, bulky and heavy weight, especially difficult to achieve lens array with high fill factor, an approach of laser beam expander by using stepped diffractive surface to approximate conic aspherical surface is proposed. It is shown that the incoming Gaussian beam with typical waist radius of 2 mm is enlarged to the designed expanding ratio of 2.8 times by the diffractive laser beam expander. By making the different sections of the surface with different steps to get two types of improved structure, which aims at keeping the stability of laser mode, the problem of fidelity and typical fabrication line-width of binary diffractive elements being not fulfilled at the same time is solved. The fidelity is raised to 95.82% with the minimum lithography line-width above 2 μm of each section, which meets the fabrication requirement of existing lithography technology. Analysis of the main errors affecting the system fidelity shows that the value is still above 95% when defocused error is -1~1 mm and decenter error is below 0.1 mm of eyepiece and 1.68 mm of objective lens, which make the system easy to install.