Abstract: The design of 400 kV intense neutron generator was presented. The electric fields of high voltage electrode and accelerating column were simulated using Poisson/Superfish code. The results show that the maximum value of electric field in the key areas is far less than the value of breakdown electric field. The program for simulating high current ion beam envelope, called IONB1.0, was developed based on high current ion beam envelope equations. The envelopes of 40 mA D ion beam were simulated in the beam transport system of 400 kV intense neutron generator. The results show that the strong focus performance from the accelerating column with two gaps high gradient electric field can effectively offset ion beam divergence caused by the space charge effect. The radius of envelope is about 3 cm at accelerating column exit. The beam transport system composed of a space charge lens and a triple quadrupole magnetic lens can focused 40 mA D beam on the target at 140 cm distance, and the beam spot diameter is less than 2 cm.