In this work, the point defect production in α-Zr at 600K. by displacement cascade with Primary Knock-on Atom (PKA) energy up to 20KeV has been investigated by molecular dynamics (MD) simulations. Especially, the influence of subcascades formation on the point defect and cluster production in 20keV cascades has been investigated in detail. The subcascade formation was seen in 36 cases of 20keV cascade simulations out of 50 cases. As the damaged areas are divided into many small areas, the maximum size of point defect cluster tends to decrease when the displacement cascade is settled. On the other hand, subcascades formation has not had an influence on the number of Frenkel pairs in final state of the 20keV cascades comparatively.