A magnetic lens is disclosed, which includes: a magnetic yoke, an exciting coil and a power supply controlling system. The magnetic yoke is at outside of the exciting coil and surrounds the coil; the exciting coil is made up of litz wires; the power supply controlling system is arranged to supply power to the exciting coil and control the flow directions and magnitudes of the currents in the exciting coil. A method for controlling the magnetic lens is also disclosed.