The wake effect on a semi-active flapping foil based energy harvester by a rotating foil is numerically investigated in this work. A NACA0015 airfoil, which is placed in a two-dimensional laminar flow, is adopted to harvest energy from the flow. It executes a forced pitching motion, which subsequently induces a plunging motion. An auxiliary smaller foil pitching about its center is arranged in the upstream of the flapping foil. As a consequence, the generated wake influences the dynamic performance of the flapping foil. At a Reynolds number of 1100 and the position of the pitching axis at one-third chord, the effects of the distance between two foils, the phase difference between the rotating motion and the pitching motion as well as the frequency of pitching motion on the power extraction behavior are examined in detail. It is shown that compared with the single flapping foil, the efficiency improvement of overall power extraction for the flapping foil together with the rotating foil can be obtained under some specific conditions. Based on the numerical analysis, it is demonstrated that the wake effect, which leads to the increase of lift force and the decrease of torque, can benefit the overall power extraction of the flapping foil.