With the geometrical optics theory of waveguide and considering quality factor or photon lifetime,general whispering gallery modes resonating condition of large size disk were obtained.The coupling coefficient between the large size disk and waveguide was investigated by the photon tunneling effect.The coupling condition of lithium niobate disk electro-optic modulator under limitation of microwave electrode width was detailed,which quality factor was about 5×105.A push-pull lithium niobate disk modulator which was coupled to a Mach-Zehnder lithium niobate waveguide was proposed to lower further driving-voltage and reduce the device size.