The porosity acquired from nuclear magnetic resonance (NMR) logging is lower than the true value in heavy oil-bearing reservoirs due to the viscosity of heavy oil, which is a great challenge in heavy oil-bearing characterization and NMR logging. To improve the porosity prediction accuracy, the effect of heavy oil should be first corrected. In this study, 10 typical core samples, which were drilled from the Hanjiang Formation of eastern South China Sea Basin, were chosen for the laboratory NMR experimental measurements under four conditions. These four conditions were initial condition, heavy oil saturated condition, residual oil condition and water saturated condition. The effects of heavy-oil volume on NMR porosity were analyzed, and a method to estimate true NMR porosity based on formation resistivity classification was established. The method was applied in our target well A14, and the true formation porosity was predicted from field NMR logging. The result showed that, the average relative error between NMR porosity and convention core derived porosity was improved from 11.19% to 4.84% by oil viscosity correct using this method. Thus the proposed method is able to effectively correct the effect of oil viscosity on NMR porosity.