Abstract: To measure high frequency auxiliary character of klystron, a high stability CW modulator is designed. The modulator's output voltage is 45 kV and output current is 9 A. 64 PSM(pulse step modulation) modules being connected in series have capability of both pulse and CW operation mode, and their output voltage can be adjusted from 500 μs pulse width to DC. Combined with step-modulation and cycle control strategy, high stability output is realized. Design of PSM module and multi-secondary transformer is given. Results of experiment prove that the modulator's output voltage stability is less than 0.1% and could turn off in less than 5 μs when arc takes place.