Display omitted •Polyamide 6 (PA6) coating with high thermal resistance was synthesized on the carbon fiber (CF) surface by in-situ polymerization. It is suitable for high-temperature molding of CF/polyamide composites.•This method is cost-effective and environmentally friendly, and the amount of PA6 coating on the CF surface can be easily controlled.•The introduction of PA6 coating made up for the defect on the CF surface and improved the tensile strength of CF.•After CF was modified with PA6 coating, the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of CF/PA6 composites were significantly increased by 49.92% and 31.94%, respectively.•The main mechanism of interfacial properties improvement is that the PA6 coating was tightly bonded with the polyamide matrix due to its similar molecular structure, and the mechanical meshing between PA6 and CF surface grooves was formed, which ensures that the resin matrix can effectively transfer the stress to CF. Commercial carbon fiber (CF) sizing agent is generally characterized by poor temperature resistance and incompatibility with polyamide resin, which adversely affects the interfacial properties of CF/polyamide composites. In this paper, we utilize the feature that anionic ring-opening polymerization of caprolactam (CPL) can synthesize polyamide 6 (PA6) in situ, introduce CPL precursor on the CF surface, and initiate polymerization to generate PA6 coating to modify CF. By changing the concentration of precursor solution, the amount of PA6 on the CF surface was effectively regulated, and the modified CF coated completely and uniformly can be obtained. The results show that PCF-20 (CF treated with 20 wt% precursor solution) reinforced polyamide composites have the best interfacial properties. Compared with the desized CF/PA6 composites, the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of PCF-20/PA6 composites increased by 49.92% and 31.94%, respectively. The interface image illustrated that the high interfacial wettability of PA6 coating on CF surface results in mechanical mesh between fiber and resin, which is the main reason for the enhancement of composite interfacial properties. Therefore, the in-situ synthesis of PA6 coating to modify CF is an effective method to improve the interface properties of CF/polyamide composites.