[Background] Classical swine fever is caused by classical swine fever virus (CSFV), which is highly contagious and fatal to pigs. Co-infection of CSFV with other swine pathogens is present in clinical practice, such as, CSFV is co-infected with foot-and-mouth disease virus (FMDV) in pigs. [Objective] The effects of co-infection of CSFV and FMDV in porcine host cells on replication of FMDV were studied. [Methods] We constructed an in vitro model of co-infection of CSFV and FMDV. The PK-15 cells were first infected with CSFV and then infected with FMDV. The replication of FMDV in FMDV-infected cells and the cells with co-infection of CSFV and FMDV was analyzed by cytopathic effect, real-time quantification PCR (RT-qPCR), Western Blot and indirect immunofluorescence assay, respectively. The CSFV proteins that included the inhibitive function on FMDV replication was subsequently screened by RT-qPCR analysis. [Results] Co-infection of CSFV C strain and FMDV inhibited the replication of FMDV. Meanwhile, the inactivated CSFV also showed inhibitive activity on FMDV replication. A detailed screening identified that CSFV C protein include the function to suppress FMDV replication. [Conclusion] We found that CSFV C strain infection could inhibit FMDV infection, and the C protein of CSFV included a function to inhibit FMDV replication.