Abstract: Magnetars are a type of neutron stars with extremely powerful magnetic fields，which also

have the strongest magnetic fields in the universe ever discovered by human. The magnetic field of

a magnetar has not only a poloidal component. Because of its high intensity，it can apply enough

stress on the shell of the star to cause shear， which leads to the twist of the magnetic field

around the stellar surface. Magnetars fit the force-free condition be- cause their

magnetic energy is dominated by magnetic pressure. Plasma pressure in the magnetosphere

is far less than magnetic pressure so it can be ignored. In this paper，we tried to find

the variable separated stream function solution around the surface of magnetar under

force-free condition and carry out numerical simulation of field lines， as well as numerical

calculation of twist angle and magnetic energy.

Key words: magnetic field, magenetar, numerical calculation