Abstract: Dissolved organic carbon (DOC) is a key component of lake carbon cycling, with its concentration directly influencing carbon flux estimation, microbial metabolism, and water quality assessment. Accurate DOC quantification is therefore critical for evaluating lake ecological functions. The combustion oxidation–non-dispersive infrared absorption method is widely used for DOC determination in water due to its high oxidation efficiency and measurement precision. This method operates in two modes: the direct method and the subtraction method. However, lakes on the Yunnan–Guizhou Plateau, owing to their unique geological formation and hydrological conditions, commonly exhibit high dissolved inorganic carbon (DIC) background, which renders DOC analysis susceptible to interference. Nevertheless, little research has addressed the applicability of these two modes specifically to plateau lake environments. To fill this gap, synthetic solutions with varying DOC/DIC ratios were prepared to evaluate the performance of the direct and subtraction methods. The results indicate that the direct mode is suitable when DOC/DIC ≤ 2:1, whereas the subtraction mode performs well when DOC/DIC ≥ 1:1. In the intermediate range (1:1 < DOC/DIC < 2:1), both modes are applicable. Based on these findings, the acidification and purging conditions of the direct method were optimized for high-DIC plateau lake water samples (DOC/DIC ≤1:3), and it was found that acidification with 200 μL of 10% phosphoric acid followed by 5 min of purging effectively removed inorganic carbon. Under these optimized conditions, the detection limit for DOC was 0.084 mg/L, spiked recoveries ranged from 95.5% to 107.2%, and relative deviations complied with standard quality control requirements. This study elucidates the differential applicability of the direct and subtraction modes under various DOC/DIC ratios and offers a theoretical foundation for selecting appropriate DOC determination methods in high-inorganic-carbon aquatic systems, exemplified by plateau lakes.