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Diamond is a highly attractive material for many applications in material science, engineering, chemistry, and biology because of its favorable properties. The preparation of large-area freestanding diamond plate can greatly reduce the production cost of diamond and promote the industrialization of diamond. In this study, the dual-magnetic field mode was introduced to optimize the magnetic field distribution in direct current arc plasma jet chemical vapor deposition (DC Jet CVD) system, and the arc plasma was effectively controlled under the coupling of electric field and magnetic field. The uniformity of the plasma distribution above the substrate surface is significantly improved, resulting in long-term stable and uniform growth of the 8-inch freestanding diamond plate. The influence of dual-magnetic field mode on magnetic field generation, arc feature, and reactive group distribution was studied. 8-inch freestanding diamond plates with an average thickness of 2.8 mm were prepared by dual-magnetic controlled DC Jet CVD. The results showed that the 8-inch diamond plate preferred a high (220) orientation, and the deviation of thickness of which was around 10 % of the averaged values. Raman spectroscopy of diamond revealed a 6.6 cm−1 of full width at half maximum (FWHM) and calculated 1.20 GPa compressive stress in the 8-inch diamond plate. By means of mechanical and thermal property measurements, the large area diamond plate shows a robust structure with the average values of three-point bending fracture strength of 987.2 MPa, and extremely high thermal conductivity of 1797.8 W/(m·k) as well. The deviation of three-point bending fracture strength and the thermal conductivity over the 8-inch diamond plate was around 7.9 % and 12.6 % of the respective averaged values. [Display omitted] • A uniform and robust 8-inch diamond plate was prepared using a dual-field DC arc CVD system. • The average three-point bending fracture strength of the 8-inch diamond plate was 987.23 MPa, and the average thermal conductivity was 1797.80 W/(m•K). • The effect of dual magnetic field mode on the distribution of arc morphology and arc plasma groups was analyzed using a high-speed camera and filter. [ABSTRACT FROM AUTHOR]