The invention discloses a directional coupler and directional coupling method for multimode optical waveguide 3dB beam splitting. The directional coupler comprises two groups of dense waveguide arrays each consisting of two or more buried rectangular optical waveguides side by side; each of the two groups of dense waveguide array is divided into an input region, a first gradual change tapered region, a coupling region, a second gradual change tapered region and an output region. The input region is connected to one end of the coupling region through the first gradual change tapered region, and the other end of the coupling region is connected with the output region through the second gradual change tapered region. The widths of the buried rectangular optical waveguides in the input region, the output region, and the coupling region, and the distance between the buried rectangular optical waveguides are adjusted by the first gradual change tapered region and the second gradual change tapered region. The directional coupler of the multimode optical waveguide based on the dense waveguide array structure realizes 3dB beam splitting in multiple modes at the same time, and has the advantages of compact and simple structure, easy processing and realization, and can be applied to mode division multiplexing integrated optical path.