A Venturi tube is typically used as the flow control element of hybrid rocket motors (HRMs) for the precise control of the flow of a liquid oxidizer, e.g., nitrous oxide, which can efficiently realize the thrust throttling of motors. The flow rate of nitrous oxide in the venturi tube was calculated for a heterogeneous non-equilibrium flow by considering the influence of the variation in the compressibility temperature. In addition, a nitrous-oxide flow rate measurement system was established to calibrate the flow rate coefficient and obtain the corresponding calculation parameters. Results revealed that upstream pressure is not affected by back pressure when the throat pressure is less than a saturated vapor pressure of 4.2 MPa at the local temperature for the venturi tube with a throat diameter of 1.65 mm; however, cavitation disappeared at a back pressure of greater than 6.35 MPa. A flow rate evaluation error of less than 6 wt% was achieved by using the experimentally determined non-equilibrium parameter in the flow rate calculation method.