The invention discloses an SOI (
silicon on insulator) based electrooptical modulator based on symmetrical and vertical 
grating coupling, which comprises a symmetrical and vertical 
coupling grating, a 3-d B 
optical beam splitter, two mode 
converters, two optical phase shift arms, an 
optical beam combiner, two 
coplanar waveguide wave traveling electrodes and an annular 
metal alignment mark, wherein the 3-d B 
optical beam splitter acts as an interface of the SOI based electrooptical modulator and a single mode 
fiber or an input end of a coupler and the SOI based electrooptical modulator; the two mode 
converters act as the connection of a wide wave guide of the symmetrical and vertical 
coupling grating and a single mode 
ridge-shaped 
waveguide; each optical phase shift arm consists of the single mode 
ridge-shaped 
waveguide and an electric structure embedded into the single mode 
ridge-shaped 
waveguide; the optical beam combiner is used for combining the light in the two optical phase shift arms which are divided by the symmetrical and vertical coupling grating into one beam, thus 
phase modulation of light is converted into 
intensity modulation; the 
coplanar waveguide wave traveling electrodes are respectively positioned on the two optical phase shift arms, form 
electric contact with the electric structure in the optical phase shift arms, and are used for the loading and transmission of 
radio frequency / micro wave electrical modulation signals; and the annular 
metal alignment mark is positioned around the symmetrical and vertical coupling grating, and is used for the alignment of the 
optical fiber in an optical grating test.