A polarization conversion system includes at least one beam splitting and combining unit having a polarization beam splitting element and a dichroic element, a dichroic unit, a polarization beam splitting unit, a first half wave plate (HWP) and a second half wave plate. The polarization beam splitting element reflects a first portion beam of the first light beam and allows a second portion beam of the first light beam to pass through. The dichroic unit reflects the first portion beam. The polarization beam splitting unit reflects a third portion beam of a second light beam and allows a fourth portion beam of the second light beam to pass through, and the dichroic element reflects the third portion beam. In addition, the first HWP is disposed between the beam splitting and combining unit and the polarization beam splitting unit, while the second HWP is disposed between the beam splitting and combining unit and the dichroic unit.