Long-term accumulation and overdose of nitrite have been proven to be an enormous threat to the ecosystems and human beings. Hence, it is of great significance to accurately evaluate nitrite by a simple and stable method. In this manuscript, Fe₂O₃/Fe₃C/Fe composite nanoparticles uniformly decorated on carbon spheres (CSs@Fe₂O₃/Fe₃C/Fe) have been developed to fabricate an electrochemical sensor for the determination of nitrite. Through in-situ thermal oxidation and reduction route, Fe³⁺ adsorbed on carbonaceous spheres were in-situ transformed into Fe₂O₃/Fe₃C/Fe composite nanoparticles on their surfaces. Based on the composition evolution with the calcined temperature, their forming mechanism was explored. Due to the synergetic effect of the adsorption/acceleration of carbon spheres (CSs) and the electrocatalytic activity of the composite nanoparticles, CSs@Fe₂O₃/Fe₃C/Fe modified GCE exhibits an excellent electrochemical sensing performance toward nitrite with a high sensitivity of 451.85 µA cm⁻² mM⁻¹, a low detection limit of 0.06 µM and a wide linear range (1–2540 μM). Furthermore, it also shows a good selectivity and stability toward nitrite. In addition, the proposed sensor was employed to evaluate the nitrite level in different pickled vegetables. The results indicate that it is a good potential application to detect nitrite in real samples.