This study explored the feasibility of char and tar formation inhibition during supercritical water gasification of sewage sludge (SS) by additive addition. Experiments were conducted in autoclave with 5 wt% additives at 400 °C for 30 min. The non-additive gasification of SS resulted in a higher char yield (12.6%) and tar yield (16.4%). In contrast, the five additives reduced the char yield (3.4-11.2%), the inhibition of char yield by additives was in the order of NaOH > K2CO3 > H2O2 > acetic acid > NiCl2. The inhibition of tar formation was limited, tar yield were 13.3-18.8% with additives. Fourier-transform infrared spectroscopy was used to determine the functional groups of char/tar, and it was observed that the spectra of char were more similar to those of hydrochar obtained in a low-temperature experiment. Model compounds of potential precursors was also tested to study the mechanism of action of additives, the results reveal that additives have different effects on char/tar formation from various components, the inhibitory effects of additives on the yield of char from humus and tar from lignin were limited. Finally, the effects of additives on gasification were also studied. The addition of additives will have an impact on the hydrogen yield and gasification efficiency, which also needs to be considered when use additive to reduce the by-products yield.