Magnetic helicity is a physics concept about magnetic field topological property.Recently,it is used in astrophysics,especially in solar physics.Now it is a very hot domain.Helicity can describes the field to pological structure.It also can express the magnetic field complex structure,for example,twist,link,braid.As a physics quantity,in perfect magnetic hydrodynamics,it is conserved.During magnetic reconnection,helicity is approximately conserved.By using this characteristic to solar atmosphere,magnetic helicity transport rate is calculated.Recently,magnetic helicity asymmetry was found in different hemisphere of the sun.In the north hemisphere,helicity is predominate by negtivity;but in the south,helicity is almost positive.Hemisphere rule in the different layers of solar atmosphere is depicted and the reasons of this rule are introduced.The newest development of helicity research all over the world and its implications in solar physics are introduced.The problems which should be resolved are pointed out.