In order to study the stability of emulsions with different water cut，the self-emulsification of crude oil and produced water with different water cut was analyzed，and combined with nuclear magnetic resonance（NMR），the stability characteristics of emulsions with different water cut in Bohai Sea A reservoir was identified. The results show that under the conditions of 65 ℃（formation temperature）and 3000 r/min rotation speed，the crude oil and water are completely emulsified at water cut of 40% and 50%，and water-in-oil emulsion（W/O）was formed，the average particle size of the emulsion was 1.905 μm and 1.487 μm，and the interfacial tension was 23.93 mN/m and 18.89 mN/m，respectively. The relaxation time and self-diffusion coefficient of the emulsion with 40% and 50% water content were the smallest，the relaxation time and self-diffusion coefficient of the emulsion with 60% water content was the second，while the relaxation time and self-diffusion coefficient of emulsion with 70% water content were the largest. When the moisture content was 40%—50%，the emulsion was the most stable. Combined with NMR technology， the smaller the average particle size of the emulsion，the higher the emulsion viscosity，the lower the interfacial tension，the smaller the relaxation time and self-diffusion coefficient，the better the emulsification effect.