The core component of three-component acceleration seismic geophone is the dual M-Z interferometer which is composed of two cascaded 3-dB coupler. The design of 3-dB coupler is particularly important to the detection accuracy of three-component acceleration seismic geophone. The structure and working principle of the three-component acceleration seismic geophone and the theory of co-directional coupling between guided modes are introduced. The structure and calculation parameters of 3-dB coupler waveguide are set. Matlab is used for the simulation of output power of 3-dB coupler. Simulation result shows that it achieves equal energy distribution in the two outputs of 3-dB coupler. Waveguide optics simulation software OptiBPM v9.0 is used to simulate 3-dB coupler. The optical power transmission diagram demonstrates that the 3-dB coupler designed realizes the equalization of optical power.