A-type granites are alkali-rich and water-poor granitoids characterized geochemically by poor Al, Sr, Eu, Ba, Ti and P. A-type granite was formed under the low pressure and high temperature conditions, and their source rocks had no choice. The essence of the A-type granites is that granitic magma melted under low pressure, and they were mostly produced in the tectonic setting of crustal extension and thinning.