In-situ upgrading of coal pyrolysis tar with steam catalytic cracking (SCC) is a promising technique to improve tar quality. In this study, Ni/Al 2 O 3 catalysts with different Ni contents were used for in-situ SCC of coal pyrolysis tar at 650°C. The results showed that Ni/Al 2 O 3 catalysts can enhance the water conversion and light tar yield. Both SCC and tar catalytic cracking can occur simultaneously over Ni/Al 2 O 3 , and the free radicals produced by tar catalytic cracking can be stabilized by active species generated from SCC to avoid excessive cracking of tar and improve light tar yield. 4Ni/Al 2 O 3 (4 wt. % Ni) exhibits the best performance, contributing to the highest water conversion of 4.74 wt. % and light tar yield of 11.90 wt. %, respectively.Correspondingly, the content of light fraction (boiling point less than 360°C) in tar sample increases from 48.0 wt. % to 67.0 wt. %, and the average molecular weight decreases from 353 amu to 253 amu over 4Ni/Al 2 O 3 . This is consistent with the increase of benzenes, phenols and naphthalenes and the decrease of long-chain aliphatic hydrocarbons, 3-and 4-ring aromatics in upgraded tar. In addition, the higher ratio of uncondensed aromatic protons to condensed aromatic protons in upgraded tar reveals the fact that 4Ni/Al 2 O 3 can promote ring-opening reaction of PAHs, which can well interpret the tar upgrading process.