The invention relates to a free space 90-degree optical mixer, which comprises a beam combining unit, an in-phase balance receiving channel, an orthogonal balance receiving channel and a receiving unit, wherein the combining unit combines a signal beam and a local oscillator beam, the in-phase balance receiving channel generates 0-degree and 180-degree relative phase shift combination beams, the orthogonal balance receiving channel generates 90-degree and 270-degree relative phase shift combination beams, and the receiving unit receives the 0-degree, 90-degree, 180-degree and 270-degree relative phase shift combination beams. The free space 90-degree optical mixer provided by the invention improves the accuracy and is convenient to adjust and install.