Abstract: Underground coal gasification (UCG) is considered a promising clean and green mining technology, particularly for utilizing residual coal resources in aging or abandoned mines to produce hydrogen-rich syngas. It also enables in-situ carbon dioxide (CO₂) storage within underground combustion cavities, contributing significantly to the efficient and clean utilization of coal and supporting China’s “dual carbon” goals. However, potential groundwater contamination caused by improper process control remains a major barrier to its industrial-scale application. This study investigates the characteristics of water pollution caused by organic compounds, such as tar, carried by syngas during UCG. Wastewater samples were collected from pyrolysis and gasification gas scrubbing processes and compared with field wastewater from UCG sites. Factor analysis was applied to evaluate the pollution profile. Results show that in the pyrolysis temperature range of 300-700 °C, gas scrubbing wastewater contains complex organic components, primarily aliphatic hydrocarbons, phenols and their derivatives, and benzene derivatives. The composition of scrubbing water during gasification is similar to that of field samples, with phenolic compounds accounting for more than 37.5%. Field measurements revealed that the chemical oxygen demand (CODCr) of extracted underground wastewater reached 413 mg/L, exceeding the regulatory limits for pollutant discharge in the coking chemical industry. Factor analysis indicates that higher pyrolysis and gasification temperatures promote the formation of water-soluble organic compounds, increasing TOC and COD levels. Aliphatic hydrocarbons are more likely to migrate with syngas and accumulate in the scrubbing wastewater through condensation. TOC and COD were integrated into a single organic pollution index, which was found to correlate positively with temperature, as indicated by increasing F-values. This study elucidates the formation mechanism and temperature-dependent behavior of water-soluble organ.