A frequency synthesizer for Hydrogen atom frequency standard which has variable six digits has been developed. The frequency of this synthesizer can be changed from 405700.0000Hz to 405799.9999Hz with a minimum step frequency of 1 ×10-4Hz. The measured results show that nonharmonic interrelated spurious output of the synthesizer is less than -85dB. The measured short term frequency stabilities σ（τ） are 2.07×10-11

2.48× 10-12 and 2.88×10-13 for averaging time of 0.1s

Is and 10s respectively. The phase drift of the synthesizer

△φ

is 28.8° when the ambient temperature varies from 20℃ to 40℃. No change in output phase of the synthesizer was observed when the supply-voltage varied either from + 16V to + 19V or from +5V to +6V. The experiments show that it can operate continuously for more than one month without troubles

and can operate normally for ambient temperatures from 0℃ to 55℃.