An apparatus of adding high vapor pressure magnesium to a steel liquid, includes a magnesium additive device and a tube furnace. The magnesium additive device includes a storage barrel, a conveying pipe, a control valve, and an insertion tube. A method of adding high vapor pressure magnesium to a steel liquid, includes placing the magnesium additive device in the tube furnace, and delivering pure magnesium particles into the storage barrel. When the temperature at the mediate lower position of the conveying pipe is increased to reach a preset value, the control valve is opened to pour the pure magnesium particles into the conveying pipe to form a magnesium vapor, and an argon regulating valve is opened to introduce the argon into the conveying pipe so as to add the magnesium vapor to a steel liquid by carrying of the argon.