Abstract: According to current magnetic and processing conditions, a gradually-changing reversal magnetic system and a practical 8 mm third-harmonic peniotron electron gun are designed. This large-orbit electron gun is designed under the operating voltage of 43.5 kV, and operating current of 1.45 A, and its cathode can be located in the axial magnetic field where its magnitude decreases gradually. It does not demand the consistency between the field lines and the contour of the beam trajectories, which reduces the design requirements and difficulty greatly. After optimization, an axis-encircling electron beam with an axial velocity spread of 4.78%, a guiding center deviation ratio of 7.18% and a velocity ratio of 2.2 is obtained, which satisfies the requirements of the third-harmonic peniotron. The large-signal simulation shows that, driven by the electron gun, the peniotron can achieve an output power of 31.9 kW with the efficiency up to 49.4%. This is an attractive result in laboratory platform.