Integrated sensing and communications (ISAC) is one of the 6G candidate technologies and is expected to further facilitate the convergence of 6G applications. As one of the deployment methods of ISAC, cooperative ISAC (CoISAC) which makes full use of the information from each different base station (BS) becomes a hot research direction in the current industry. However, CoISAC requires high synchronization performance among cooperative BSs where a small time-frequency synchronization error can lead to large variations in the distance-velocity estimation results. To address this problem, this paper proposes an inter-BS synchronization scheme based on the reference path calibration. Specifically, based on the actual distance and velocity of the reference path, we obtain the synchronization offset between BSs. Then we verify the calibration capability of the scheme through simulation, the proposed scheme can correct the inter-BS synchronization errors and any frequency offset when exceeding the maximum unambiguous speed. Finally, we build a CoISAC experimental platform in an indoor scenario and verify the proposed scheme in a low-frequency band. This work provides the basis for subsequent CoISAC studies.