For the application of low noise and high linearity in microwave photonic link, the influence of waveguide cross coupling effect of M-Z electro-optic modulator on harmonic suppression ratio is studied in this paper，Firstly, through the joint simulation of OptiBPM and MATLAB, it is found that the cross coupling effect between waveguides will make the operating points of RF electrode and bias electrode cheaper, and then reduce the harmonic suppression ratio，Secondly, the phenomenon is verified by a special lithium niobate modulator，At last, a method to detect the cross coupling effect of M-Z electro-optic modulator is proposed, This discovery not only breaks the blind area of the influence of waveguide structure on harmonic suppression ratio, but also provides a certain reference for the development of thin film lithium niobate modulator for microwave photonic technology..