On the basis of the analysis of the flow mechanism of throttle tubes, the pressure drop model of the single-phase flow through the throttle tube is derived from the mass, momentum and energy conservation equations. Good agreement is found between the pressure drop model and the experimental results. The accuracy of the model gained in the experiment is highly satisfactory. Furthermore, it is pointed out that the main resistance loss of the single-phase flow through the throttle tube is that of the abrupt-enlargement, and the inlet abrupt-contraction resistance should not be neglected at all. Besides, the frictional resistance must be taken into account for the longer throttle tubes.