Precise quantification of soil carbon is the prerequisite for the study of soil fertility and carbon sequestration in agriculture. This study selected eight representative soil/sediment samples with different contents of organic carbon (OC) and inorganic carbon (IC), and compared the accuracies of three analytical methods, i.e., wet oxidation method, dry combustion method with acid washing, and indirect method. The results showed that, for soil and sediment samples with OC contents of 3.4–11.5 g/kg, wet oxidation method had the lowest mean relative error (0.32%–4.72%) and the lowest coefficient of variation (0.40%–1.22%), while the indirect method had the highest mean relative error (5.16%–20.46%) and the highest coefficient of variation (0.60%–7.00%). These results indicated that if a stable and accurate heating temperature was maintained, the measured results of wet oxidation method, the simplest method, are stable and accurate. Due to a difficult dissolution of inorganic carbon in the dry combustion with acid washing, or difficult total combustion of inorganic carbon in the indirect method, these two methods had a positive error of organic carbon and the coefficient of variation was also higher for most samples.