Computational fluid dynamics (CFD) methodology is successfully applied to investigate the flow field of a turbo booster pump (TBP) in slip and continuum flow. This pump was developed through cooperation between Kashiyama Industries Ltd. and the Precision Instrument Development Center with the goal of replacing the mechanical booster pump. The rotor of the TBP is equipped with five spiral blades that are hybrids of Becker's turbo blade, Holweck's helical-grooved element, and a connection area with a transition spiral-grooved segment. To simplify the computational model and to shorten the calculation time, only one single flow channel is calculated. The full computational domain is linked by five regions including a clearance area between the blade tip and the casing, a vacuum chamber upstream of the inlet of the pump, and an exhaust region of the pump. The experimental measurement is made according to Japan's JVIS-005 standard. The flowmeter method is adopted here to calculate the pumping speed. Comparisons of measured and calculated inlet pressure show that the calculation is quite accurate when the pumping fluid is in slip and continuum flow. But, in transition how, the continuum model of CFD proves to be unsuitable for calculating such rarefied gas. Because of variation of the averaged pressure, the tendency of pressure to increase is quite linear in the regions of the turbo blade and the connection design area. In the helical-grooved design area, the trend is similar to an exponential curve. According to the results of the velocity field, in lower pressure the velocity field of the flow channel is quite smooth, flowing from the pump inlet to the outlet. But in higher pressure, the velocity field is complex. There are vortices within the flow channel. And the backflow from the pump to the vacuum chamber is more serious, and will harm the pumping efficiency of the TBP.