The catalytic decomposition of hydrazine (N2H4) could release lots of energy, and there have been extensive studies on metal surfaces as catalysts for this process. Here, we first reported the detailed mechanisms of adsorption and catalytic decomposition of N2H4 on metal-free nanomaterials with the carbon-based SiC3 siligraphene (g-SiC3) as substrate. By using density functional theory methods, it is found that N2H4 molecule chemically adsorbs on g-SiC3, with the anti configuration as the most stable one. The following analysis show orbital hybridization between N2H4 and g-SiC3. By analyzing the potential energy surfaces, it is revealed that the optimal decomposition pathway of N2H4 on g-SiC3 is mainly the pre-adsorbed NH2 intermediates assisted intermolecular decomposition, with N-2 and NH3 as products. The rate determining step of the optimal decomposition pathway is *(N2H3 + NH2) -> * (NNH2 + NH3), which is different from the decomposition pathway of N2H4 on the extensively researched metal surfaces. Our results provide rational principles of metal-free carbon-based catalysts for adsorption and catalytic decomposition of N2H4.