The invention provides an optical mixer of free space. The optical mixer is characterized in that: a polarizing splitter respectively separates beam S and beam L into horizontal polarization beams Sx and Lx as well as vertical polarization beams Sy and Ly; at least one birefringence crystal divides the horizontal polarization beams Sx and Lx as well as the vertical polarization beams Sy and Ly into beams Sxo, Sxe, Lxo, Lxe, Syo, Sye, Lyo, and Lye; and then beam Sxo+Lxo, beam Sxo-Lxo, beam Sxe+jLxe, beam Sxe-jLxe, beam Syo+Lyo, beam Syo-Lyo, beam Sye+jLye, beam Sye-jLye are combined by a light combiner and are output by a port. The advantages of the invention are as follows: an optical mixer of free space is allowed by utilizing a birefringence crystal, so that beams with different polarization can be separated well in free space; and the optical mixer has a compact structure and good stability.