Conversion of syngas to useful chemicals is a hot topic and the supported Co catalyst plays a very important role in this process. To prevent the aggregation of Co crystalline, it is suggested to construct a core-shell catalyst Co@C, in which carbon creates a confined microenvironment to encapsulate the active Co sites. Here, we proposed a simple but effective method to fabricate a core-shell catalyst Co@C. Different adsorbability to reactants and the confinement effect induced by the microenvironment around active sites have been studied. The influence of the carbon material property such as graphitization degree on catalysis performance was studied in detail. Different temperatures and atmospheres in the calcination process were demonstrated to influence the graphitization degree of the carbon shell. The graphitization degree could affect the adsorbability to hydrogen on Co@C, which had a significant impact on CH4 selectivity in syngas conversion. The CH4 selectivity on Re-Co@C-750 was three times higher than that on Co@C-750 because of a decreased graphitization degree of carbon.