Traditional high-order stepped spiral phase plate that is difficult to fabricate on the fiber end face and easy to damage during the application. In order to solve these issues, a design scheme of flat spiral phase plate fabricated on the fiber end face is proposed. This kind of phase plate has the same height at different azimuth angles, and different topological charges are obtained by controlling the refractive index at different azimuth angles. The Fresnel diffraction theory is used to conduct a detailed theoretical analysis of the vortex electric field distribution and light intensity distribution generated by this scheme. The finite difference time domain method is used to simulate the propagation process of the fundamental mode Gaussian beam in the fiber after passing through the flat spiral phase plate with different topological charge. The results show that this kind of flat phase plate on fiber end face can generate high-quality vortex beams for both high-order and low-order modes, which effectively overcomes the shortcomings of traditional stepped spiral phase plate.