A critical venturi nozzle with conical inlet is designed on account of the machining difficulty for the structure with circular arc entrance under small scales such as diameter less than 200μm The numerical simulation method is adopted to optimize the structure of the cirtical nozzles. The impact on the discharge coefficient of the nozzle is analyzed caused by the various geometrical parameters such as inlet angle, length of entry, throat length, divergent angle and length of divergent section. The simulation results show that the discharge coefficient is influenced by the length of the throat and inlet angle greatly. The length of the throat has negative linear correlation with discharge coefficient. The discharge coefficient may reach maximum value when the inlet angle is 17.5°. The discharge coefficient grows up as the diffusion angle increases obviously until the angle value is up to 3°. A calibration facility based on the critical nozzles is developed for the small gas flow. The performance of the facility is investigated by experiments. Test results show that the device can calibrate flowmeter ranged from 0.01 to 1.0m³/h. The expanded uncertainty of the device is 0.22%.