White blood cells (WBCs) are a crucial component of the human immune system. WBCs contain invaluable information about the health status of the human body. Therefore, separating WBCs is indispensable for the diagnosis of many diseases in clinical setting. The low ratio of WBCs to red blood cells in whole blood has made the isolation of WBCs challenging. As the conventional single-stage microfluidic technology cannot provide sufficient separation purity. We used a cascaded inertial microfluidic chip by consecutively connecting two sinusoidal channels to enhance the purity of WBCs after single processing. The improvement was in part due to the diversion of the sample at the end of the first stage separation, resulting in a lower flow rate in the second stage of processing within the cascaded device. We embedded concave micro-obstacles in sinusoidal channels to adjust their effective working flow rate range and enable the proper operation of both channels simultaneously. Using polystyrene beads mixture (5 and 10 μm) with a primary ratio of 1,000 to 1, a single processing step through our cascaded chip improved the purity of 10-μm particles with more than three orders of magnitude of enrichment (from 0.08% to 99.83%) with a flow rate of 560 μl/min (Re = 77). Using diluted whole blood (×1/10), we achieved 307-fold enrichment of WBCs (0.14% to 43.017%) in a single process which was accompanied with ∼3 orders of magnitude background removal of RBCs (from 4.8×108 to 5.7×105 counts/mL). This cascaded manner chip has the capacity to achieve high-efficiency separation of blood cells for clinical diagnosis.