In the microwave band，a wideband frequency synthesizer with small step and low phase noise is commonly implemented by Direct Digital Synthesis(DDS) driving Phase Locked Loop(PLL). However，it is difficult to realize low spurious(especially the near-end spurious) at the same time. Based on the studies of DDS with low spurious technology，an improved wideband frequency synthesizer is introduced，which could effectively improve the spurious suppression indexes. The proposed synthesizer is provided with the frequency range from 4 GHz to 8 GHz with the stepping of 100 kHz；and its spurious suppression index is better than -70 dBc in the whole range.