This paper analyses the electron emission from cathode stick and multi-beam cathode rod of MBK and investigates the transport efficiency with different radius and drift tube length. The results show that with compact constructure the electron emission from cathode stick and rod under high voltage affect the multi-beam transmission efficiency obviously. Especially, the beam bombing on the wall of tube will degenerate the repetition capacity of multi-beam klystron. To resolve these problem, a combined magnetic system is designed and fabricated, which can reduce the stick and rod emission by pulling the multi-beam cathode out of the high electric zone. Then a multi-beam diode with combined magnetic system is designed. Calculation and simulation results show that beam emission from the cathode stick and the rod can be decreased evidently, furthermore the cathode stick can be removed. In addition, the magnetic distribution can ensure high efficiency and stability of multi-beam transport. At present, the multi-beam diode with combined magnetic system is being experimented.