The energy harvesting of a hydrokinetic turbine based on tandem flapping hydrofoils is investigated experimentally and numerically. The mechanisms with crankshafts and connecting rods are adopted to convert the oscillatory motions of the hydrofoils into rotary motions. The hind hydrofoil performs oscillatory motion in the vortex flow of the fore hydrofoil. The longitudinal spacing L-x between the two hydrofoils is fixed while the phase difference epsilon between the flapping motions of the two hydrofoils can be shifted from -pi to pi. The global phase shift Phi, which is the combination of L-x, epsilon and the Strouhal number St, is adopted to describe the vortex interaction modes. The effects of St and Phi on the energy harvesting have been tested. Experimental data show that the highest hydrodynamic efficiency of single hydrofoil is 25.2% at St = 0.235. The optimum parameters for the energy harvesting of tandem hydrofoils have been found. The highest hydrodynamic efficiency is found at St = 0.24 with Phi/2 pi approximate to 0.3. Numerical simulations are performed to study the vortex interaction modes of these typical cases. The beneficial vortex pattern and the detrimental vortex pattern, which result in the highest and lowest efficiency of the turbine, have been investigated. (C) 2019 Elsevier Ltd. All rights reserved.