Molecular sieve (also called as synthesized zeolite) is a kind of silica-aluminates with micro-porous crystal. It is a frame-work which is constructed by SiO2 and Al2O3. Molecular sieve has very similar aperture value (only several A) and large surface area. There are relevant positive ions in its lattice to balance the negative ions in the crystal frame. Different molecular sieves have different shapes and different aperture values. We could separate different kinds of molecular by selectively using different molecular sieves. Water is a sort of polar molecule, molecular sieve type A has dramatic affinity for water. For the above reasons, molecular sieve is considered as the most ideal desiccant. The molecular sieve here mentioned is type 3A which is only for adsorbing water. The water adoption depth of molecular sieve 3A is above several dozen times higher than any other applicable desiccants.