Free space optics (FSO) can achieve higher throughput due to a more concentrated energy by narrower divergence angle, compared with radio frequency (RF) communications. However, the receiver still cannot fully utilize all of the beam power, since the receiving aperture is always much smaller than the spot size at the receiving end. To capitalize on wasted power, we propose to add a Mirror-based Intermediate Reflector (MIR) in an FSO systems inspired by intelligent reflective surfaces, where the MIR can reflect part of the original lost energy to the receiving end. Based on geometric optics, the placement rules of the MIR are first obtained, where there are four criteria for the location and the angle. We further derive the expressions by regional integration method to quantify the power gain introduced by MIR. The derivation considers whether the reflected beams of the MIR all satisfy the case of incidence on the receiving target surface at an angle smaller than the field of view, and therefore the discussion is developed in two cases. Simulations show the feasibility of MIR, where the power gain is quantitatively illustrated. The results show that the reflection scheme can improve the received power gain, which provides a realizable approach to enhance the availability of FSO.