Because of the complex factors in bulk water, the effect of LIBS on the analysis of liquid sample is limited in liquid sample. Under this circumstances, a new method of LIBS assisted by ultrasonic nebulizing technology has been introduced. According to this method, the liquid sample is transformed into dense droplets by ultrasonic nebulizing technology, and these dense droplets are subjected to the analysis of LIBS later. Based on this thought, a detection experimental system was established, composing of ultrasonic system, 1 064 nm ND:YAG laser, and ICCD system. A series of experiments and analysis were carried out to detect the magnesium element dissolved in pure water using the detection experimental system mentioned above. The results showed that even with a very low laser pulse energy (30 mJ), the signals of LIBS still have a long lifetime and a high signal to background ratio. The limit of detection for magnesium element could reach as low as 0.242 ppm. At the same time, the electron density of plasma was calculated utilizing the Hα line to give the evolution features of plasma induced by this new method.