An energy extracting technology based on tandem oscillating hydrofoils from flow field is presented in this paper. Numerical simulation were performed to optimize the spatial arrangement for tandem oscillating hydrofoils, the position of the downstream hydrofoil relative to the wake vortex shed by the upstream hydrofoil is seen as critical. Optimized position of the two hydrofoils improves the energy extraction performance through positive interactions between the downstream hydrofoil and the wake vortices of the upstream hydrofoil, and the highest energy extraction efficiency reaches 53.8%. The downstream hydrofoil has a slight impact on the energy extraction performance of the upstream hydrofoil, and the contribution from the upstream hydrofoil is usually slightly inferior to the single hydrofoil results. Leading edge vortex (LEV) is formed and shed from the upstream hydrofoil, which is seen as critical in the energy transformation between the fluid and the energy extraction device. For different reduced frequencies, the energy extraction of a single hydrofoil is heavily influenced by the dynamics of vortex forming and shedding. The investigation was also undertaken over a wide range of kinematic parameters, including hydrofoil separation distance and reduced frequency. Results reveal that energy extraction of tandem oscillating hydrofoils shows better performance than a single hydrofoil for optimal reduced frequency and suitable hydrofoil separation distance.