Method for preparing olefin polymer in a loop reactor, said loop reactor comprises a first outlet for withdrawing polymer slurry from the loop reactor, and a second outlet for withdrawing a polymer slurry from the loop reactor, wherein the first outlet is located such that polymer slurry is withdrawn having a concentration of polymer which is equal or higher than the average concentration of polymer in the loop reactor, and the second outlet is located such that polymer slurry is withdrawn having a concentration of polymer which is lower than the average concentration of polymer in the loop reactor, the method comprises the steps of supplying olefin monomers and a catalytic system to the loop reactor to form a polymer slurry in the loop reactor, and controlling the total amount of polymer and/or the total amount of polymer slurry withdrawn from the loop reactor by adjusting the ratio of polymer slurry withdrawn through the first outlet and polymer slurry withdrawn through the second outlet.