The design requirement and principle of the deflection magnet for Magnetron and Penning H- ion source are discussed. It is proved that there exists a maximum emittance for the beam that may be transformed by the magnet into a state with equal Twiss parameters of αr =αy and βr =βy, which is the requisite condition to get a minimum emittance at the entrance of RFQ after transporting by a LEBT with solenoids. For this maximum emittance, the corresponding magnetic field gradient index is 1. The design requirement and principle of the steering magnet for Magnetron and Penning H-ion source are discussed. It is proved that there exists a maximum emittance for the beam that may be transformed by the magnet into a state with equal Twiss parameters of αr = αy and βr = βy, which is the requisite condition to get a minimum emittance at the entrance of RFQ after transporting by a LEBT with solenoids.