A novel approach to achieve a large-orbit electron beam is demonstrated using a gradually-changing reversal magnetic field. On the basis of analyzing the general regularities of electron movement and various factors which lead to eccentricity and velocity spread in the gradually-changing reversal magnetic field, we design a large-orbit electron gun. Different from the traditional three-step method, our design does not pursuit the formation of thin tubular electron beam and the utilization of mutation reversal magnetic field, which reduces the difficulties in structure complexity and tube-making process. In addition, the cathode emission band can be placed in the axial magnetic field before the magnetic reversal point where its magnitude decreases gradually, by controlling the angular momentum difference between every trajectory starting points and using the offset effect of various unfavorable factors to reduce eccentricity and velocity spread. The simulation results are consistent with the theoretical analyses, which shows that the beam quality can be improved remarkably by fine-tuning electromagnetic fields, confirms that the efficiency and the applicability of the adjusting method we proposed, and provides a new technical way to obtain a high-quality large-orbit electron beam for high-efficiency large-orbit millimeter-wave devices.