Abstract: A new travelling light modulator using single ridged waveguide as microwave circuit is designed.The electric field distribution、cut-off wavelength、bandwidth and characteristic impedance with d＜1.00 mm are analysed and calculated,the transition between the ridged waveguide and the standard rectangular guide is designed.The results show that the microwave applied field is made good use of,the modulation effciancy can be heightened;the band of single working mode can be widened;the characteristic impedance can be lowered;the transition can be easily connected with the standard rectangular guide and working at single mode.The feasible parameters of single ridged waveguide are a=13.00 mm,b=10.16 mm,d=0.50 mm,and s=1.00 mm.