This paper proposes two simple approaches for in situ generation of nickel nanoparticles in the carbon matrix of rice husk char (RHC) via incipient wetness impregnation and pyrolysis. The nickel oxides (i.e., NiO) in the RHC matrix can be preliminarily in situ reduced into metallic nickel (Ni-0) by carbothermal reduction. Furthermore, due to the strong reducibility, the addition of NaBH4 could significantly promote the formation of crystalline metallic nickel (Ni-0) nanoparticles. Interestingly, NaBH4 added into the Ni2+ solution can generate NaOH after catalytic hydrolysis, which contributed to the increase of BET surface area and the change of char pore structures by wet activation. More importantly, RHC Ni catalysts exhibited good performances on catalytic tar conversion with syngas upgrading during copyrolysis with biomass, especially in terms of tar conversion efficiency up to 96.5%. Consequently, in situ catalytic conversion could inhibit the micromolecular nascent tar polymerization, thereby reducing the macromolecular tar formation.