The invention provides a preparation method of a Co-N-C/SiO2 composite nanometer catalyst. The method comprises the following steps: uniformly mixing and sintering cobalt metal precursor salt, a nitrogen-containing organic carbon source and mesoporous silica, so as to obtain the Co-N-C/SiO2 composite nanometer catalyst. In the catalyst, ultra small cobalt metal nanometer particles can be properly stabilized in a carbon layer, proper pore structures and pore diameters can further effectively promote the diffusion and the separation of reaction matrixes and products, and the catalyst has good dispersibility in an aqueous system and further is easy to recycle. The preparation method does not need any solvents, can obtain the composite nanometer catalyst by one step, and is easy and practical. The composite nanometer catalyst prepared through the method does not need any complex pretreatments, can be applied to hydrogen production-hydrogen addition coupling reaction, and shows excellent catalytic activity and stability for preparing phenylamine and derivatives thereof though nitrobenzene compounds.