Abstract: Underground coal gasification (UCG) is recognized as a clean and green coal mining technology. It is particularly promising for utilizing residual coal resources in abandoned and aging mines to produce hydrogen-rich gas, while simultaneously enabling CO₂ storage in the combustion cavity. This technology is of great significance for promoting the efficient and clean utilization of coal and for achieving China’s “dual carbon” strategic goals. However, the potential risk of groundwater contamination due to improper process control remains a key constraint restricting the industrial application of UCG. In this study, gas washing water from coal pyrolysis and UCG experiments was collected. Employing factor analysis in conjunction with wastewater pollution indicators, this research elucidates the contamination characteristics of groundwater resulting from the migration of organic pollutants in tar via high-temperature coal gas. The results show that at pyrolysis temperatures ranging from 300 to 700 °C, the organic components in the pyrolysis gas washing wastewater are complex, mainly including aliphatic hydrocarbons, phenols and their derivatives, and benzene and its derivatives. The characteristics of the laboratory-generated UCG washing wastewater were found to be similar to those of actual wastewater sampled from UCG sites, with phenols accounting for the highest proportion, exceeding 37.5%. The chemical oxygen demand (COD) of water pumped from the underground UCG cavity was 413 mg/L, which exceeds the relevant discharge standards (e.g., for the coking industry). As the pyrolysis and gasification temperature increased, a greater amount of water-soluble organic matter was generated. Consequently, the concentrations of total organic carbon (TOC) and COD in the gas washing wastewater increased. More aliphatic hydrocarbons migrated with the coal gas and were enriched in the wastewater upon condensation. Through factor analysis, TOC and COD were combined into a single organic pollution index, which was found to be temperature-dependent. The water pollution degree, represented by the F-value, increased with rising temperature. By elucidating the formation mechanisms of water-soluble organic pollutants during UCG, this work provides a theoretical basis for groundwater pollution prevention and control in UCG processes.