Laser-induced breakdown spectroscopy has been used to detect sodium (Na) in water. Laser-induced breakdown was formed by dual-pulse and crossed beam Nd:YAG lasers on a water film. To improve the detection sensitivity, the fluorescence intensity dependence on timing between laser pulses, the delay time of fluorescence detection timing, the gate width of fluorescence detection period, and the laser energy were investigated. Under the optimized conditions, the detection limit of Na in water was achieved in the range of 0.1 parts per billion. The developed system is applicable for quick and supersensitive detection of Na atoms in water.