The characteristics of ion beam extraction were investigated with the CST (Computer Simulation Technology) studio suite in order to focus the beam to a spot as small as possible. The simulation process can provide a good study for optimizing the extraction and focusing system of the small sealed tagged neutron tube. The spot and the area distribution of the deuterium ion beam were affected in a three cylinder lens system by some parameters such as the distance of between three electrodes, the voltage of extraction, the voltage of acceleration, the extraction chamfer angle, the electromagnetic field of the ion source. The results showed that the electrodes position, voltage magnitude, extraction chamfer angle, plasma density and magnetic field produced by permanent magnetic ring can effectively focus on the extraction beam. The the spot size in the target can be reduced to 1mm radius which is the best parameter for tagged neutron tube. The applied parameters in all simulations are determined based on the required features of NT50 neutron tube developed in Northeast Normal University. Also, permanent magnet and electrostatic shroud can effectively suppress secondary electrons, and the suppression effect of the electrostatic shroud is better.