Abstract: To address the significant bias in the determination of organic matter (OM) in solid waste with high inorganic salt content using the loss-on-ignition (LOI) method, this study focused on typical high-carbonate phosphate tailings to systematically analyze the interference mechanisms of the LOI method and evaluate the accuracy of the subtraction method for correcting the measurement results. By using OM determination and multi-scale characterization techniques, including thermogravimetry coupled with Fourier-transform infrared spectroscopy–gas chromatography/mass spectrometry (TG-FTIR-GC/MS), we identified and quantified the sources of mass loss during the ignition process. The results indicated that the OM content measured by the LOI method had poor precision and was significantly overestimated. Pyrolysis analysis confirmed that the CO₂ released from carbonate decomposition was the dominant source of mass loss (accounting for more than 94%), while the contributions of crystalline water and sulfides were relatively small. Based on this, we proposed adopting the subtractive correction method to correct the OM content determined by the LOI method. This method quantifies the change in inorganic carbon (IC) content before and after ignition to calculate the amount of CO₂ released from carbonate decomposition, which is then subtracted from the total loss on ignition. The corrected actual OM content in the phosphate tailings ranged from 0.20% to 0.23%, showing no significant difference from the results measured directly by a total organic carbon (TOC) analyzer (P>0.05).