As a novel propulsion technology, ultrasonic electric propulsion is mainly applied to micro-satellite platforms (<10 kg). In this work, effects of vibration frequency and typical operating conditions on extraction process of charged droplets in ultrasonic electric propulsion are investigated by an ultra-high speed imaging technique. A long-distance microscope coupled with an ultra-high speed camera (NAC HX-6) is also used. A solution of formamide and lithium chloride is employed as propellant. Experimental results show that mean diameters of charged droplets are 43.3 ± 4.2 µm (120 kHz), 76.2 ± 5.8 µm (60 kHz), and 101.4 ± 7.1 µm (25 kHz), respectively. Besides, the theoretical diameter values are calculated, which are close to experimental ones and both values decrease with the increase of vibration frequency. Meanwhile, experimental results indicate that the diameter of charged droplets increases as the propellant flow rate rises, and such effect is obvious when the flow rate is ranging from 3 to 10 ml/h. It is also found that charged droplets are not uniformly accelerated and their motion directions are also divergent.