Liquid nitrogen refers to nitrogen in liquid state as its name. It is inert, colorless, odorless, non-corrosive, non-flammable and low in temperature. Its boiling point is 77°K (-196 ℃). In the normal atmospheric pressure and at temperature below minus 196 degrees Celsius, it will become liquid nitrogen. Skin will frostbite in case of keeping contact with liquid nitrogen. At the atmosph-eric pressure, excess nitrogen arising from vaporization may reduce oxygen in the air so as to cause hypoxia suffocation. Liquid nitrogen is usually used for fracturing and acidizing operation for gas field development so as to lower the density of liquid and increase flowback of fracturing fluid.