In this study, we present a double-layer NiO/β-Ga2O3 p-n heterojunction diode, which exhibits high performance with breakdown voltage, on-resistance and turn-on voltage. β-Ga₂O₃ has been attracting great attention as one of the excellent materials for next-generation high-performance electronics, owing to its high Baliga’s figure-of-merit (BFOM), carrier mobility, critical breakdown field strength and low-cost, simple growth methods. β-Ga₂O₃ based unipolar devices, especially Schottky barrier diodes, have been developed rapidly; however, the lack of p-type doping in Ga₂O₃ remains a challenge for bipolar devices, which can more generally be utilized in practical applications. One of the possible strategies is to introduce a p-type material to construct p-n junctions with n-type β-Ga₂O_3. The NiO turns out to be a feasible p-type material that has a wide bandgap, adjustable doping concentration, intrinsic p-type conductivity and diverse deposition techniques to form a uniform film, such as sputtering, PLD and thermal oxidation. In this study, we fabricated the diodes from depositing Ti/Au for the back Ohmic contact metal and annealing at 550℃ in N₂, followed by sputtering two NiO layers with different doping concentrations by tuning the ratio of Ar/O₂. The Ni/Au contact metal was deposited onto the NiO layer after annealing at 300℃ under O₂ ambient. Compared to typical NiO thicknesses of 300 to 500 nm, we turned to another trend with ultra-thin layers of NiO. The top layer NiO thickness is 10nm while the bottom layer of NiO thickness varies from 10 to 80 nm. Through the design of the ultra-thin (20 nm) double-layer NiO structure, the breakdown voltage (V_b) is substantially improved to 4.7 kV and demonstrates the on-resistance (R_on) of 11.3 mΩ·cm², which leads to a figure-of-merit (V_b²/R_on) of 2 GW/cm². The high V_b is attributed to the structure of both the double-layer and the guard-ring designs. From the TCAD simulation, the peak of the electric field locates at the edge of the diodes. Increasing the doping concentration of NiO layer contact with Ni/Au can reduce the electric field on the edge of the Ohmic contact. On the other hand, the low doping concentration of the NiO contact with β-Ga₂O₃ can effectively move the electric field crowding on the edge to the inside of devices. In addition, the extension guard ring can also spread the electric field crowding. This work provides a desirable design strategy for p-n heterojunction, which has the highest breakdown voltage among all Ga₂O₃-based p-n diodes.