To overcome the dependence of the plasma extraction on siphon hydrophilicity in a centrifugal microfluidic chip and to ensure long-term stability and reliability of the microfluidic chip, a compressed air reservoir connected to the blood separated reservoir was proposed to separate the blood in blood separated reservoir at high rotating speeds. Through lowering the spinning speed, the plasma in the separated reservoir was pumped toward the CD-like chip center to release the stored air pressure produced by the rotation at high spinning speeds. Based on the thermodynamics of the isothermal gas, the pneumatic pumping method was modeled, and the model was confirmed by analyzing pumping positions and air compression versus spinning speeds. By using the layered Polymethyl Methacrylate(PMMA)as materials, the centrifugal microfluidic chips for plasma extraction were fabricated with a CO2 laser process technique and the volume of compressed air and the blood position at siphon were tested at different rotational speeds. Experimental results demonstrate that the achieved volume of compression is 8.7μL and the siphon valve effectively inhibits whole blood to overflow the crest of siphon at 4 000 r/min speed. With lowering the spin speed to 1 000 r/min, the release of compressed air overcomes the centrifugal force and drives the plasma to flow past the crest of siphon. Then the plasma extraction is realized in quantitation.