The ability to efficiently fractionate blood into its individual components is crucial for therapeutic and diagnostic purposes. Here, we developed an inertial microfluidic system for cell focusing and blood plasma separation. First, polystyrene beads and blood cells were used to investigate the inertial focusing performance of a single slanted spiral microchannel as a function of particle size, flow rate, and the haematocrit (HCT) concentration. Next, we built a high-throughput system consisted of 16 interconnected spiral channels which can process diluted samples at 24 mL/min. The proposed multiplexed system surmounts shortcomings of previously-reported microfluidic systems concerning throughput, yield and operation efficiency.