An　effective　design　method　of　freeform　micro　lens　array　is　presented　for　shaping　varied　laser　beams　into　prescribed　rectangular　illumination.　The　variable　separation　mapping　is　applied　to　design　concave　freeform　surfaces　for　constructing　a　freeform　lens　array.　Several　dedicated　examples　show　that　the　designed　freeform　optical　lens　array　can　achieve　a　prescribed　rectangular　illumination　pattern,　especially　without　considering　the　initial　states　of　incident　laser　beams.　Both　high　collection　efficiency　and　good　spatial　uniformity　can　be　available　simultaneously.　Tolerance　analysis　is　also　performed　to　demonstrate　that　this　optical　device　can　well　avoid　fabricating　difficulty　in　actual　applications.　©　2016,　Tianjin　University　of　Technology　and　Springer-Verlag　Berlin　Heidelberg.