Abstract: A composite guide magnetic field system for axial foilless diode was designed, which included a solenoid, a permanent magnet and a soft magnet. The cathode emission region was put outside the solenoid magnet, to get smaller inner radius of solenoid and lower electric field on cathode rod surface. The permanent magnet and the soft magnet compensate magnetic density of the cathode emission region, to meet the requirement for stable transmission. Weight and power of the guide magnetic field system were both reduced more than 40% as compared with the system using a classical solenoid magnet. PIC simulation on transmission characteristic of electron beams in the composite guide magnetic field system was studied, which proved that axial electron beams could be stably transmitted in the system.