The invention provides a separating method for circulating tumor cells. The separating method includes the following steps that 1, a blood sample and a PBS solution are mixed uniformly to be put on amembrane upper layer of a porous membrane centrifugal tube so as to be subjected to density gradient centrifugal separation; 2, the solution on the membrane upper layer is taken out and put into a first centrifugal tube, centrifuging is conducted, supernate is removed, and therefore a solution containing target cells is obtained; 3, CD45 and CD15 mixed magnetic beads are added into a second centrifugal tube, the solution containing the target cells is transferred into the second centrifugal tube, uniform mixing is conducted, and vibration hatching is conducted at the temperature of 2-8 DEG C;4, hatching mixed liquor is put on a magnetic frame for standing, the supernate containing target cells is transferred into a third centrifugal tube, centrifuging is conducted, the supernate is removed, and purified circulating tumor cells are obtained. According to the method, density gradient centrifugation and immunomagnetic bead negative enrichment are combined, the tumor cells are obtained through separation from peripheral blood, the period of separation is short, the quantity of needed samples is small, the tumor cells are enriched by a high level, the recovery rate of the tumor cells is up to 70% or above, and the residual quantity of leukocytes is smaller than or equal to ten thousands.