The modulation phase was solved based on the geometric steady-phase method, the Fourier transform was carried out through Fourier lens, and the corresponding flat-top beam was focused at the rear focal plane, so that the circular Gaussian beam was shaped into a square flat-top beam, in which the optical path parameters had a great influence on shaping quality. In the experiment, it was found that the square flat-top beam was affected by the zero-order light and had poor uniformity. Therefore, the causes of the zero-order light were analyzed in detail and effective solutions were proposed. The zero-order light in the experimental results was directly removed by stacking the shining grating. The experimental results show that the energy utilization rate of square flat-top beam is 72.3%, and the beam uniformity is up to 97.2%, which is better than the traditional shaping method. Furthermore, the quality of beam shaping by geometric steady-phase method are further analyzed for three factors: the offset of hologram, out-of-focus and waist radius of incident beam, which provides convenience for the application of beam shaping by geometric steady-phase method.