Array research of tidal turbine is critical to make it cost-effective and commercializing. There are many studies of turbine arrays, which mostly focus on a conventional horizontal-axis turbine. While a ducted turbine is also available for commercial application, which draws little attention. In this paper, the tandem arrangement of ducted horizontal-axis turbines is studied based on the flume experiment and the large eddy simulation (LES). The turbine's performance changes are investigated under the 5-15D (D represents the turbine diameter) tandem distance. Research results show that the downstream turbine's performance degrades significantly, and its efficiency drops to roughly half of the upstream turbine when the tandem distance is 5-10D. When the distance increases more than 10D, the downstream turbine performance can reach 80% of the upstream turbine. As the distance increases, this performance recovery becomes less significant. In terms of load fluctuation, with the interaction of the upstream wake and the surrounding flow field, a spiral oscillation wake region will be formed about 10D behind the upstream turbine. Therefore, with the increase of the tandem distance, the load fluctuation of the downstream turbine does not gradually decrease as a result. This fluctuation is more than twice that of the upstream turbine, and the maximum can reach 5 times.