An electro-optic switch was developed based on the lithium niobate on insulator （LNOI） platform. The structure of the LNOI waveguide， multimode interferometer （MMI） coupler and metal electrodes were carefully designed. The fabricated electro-optic switch chip was coupled with optical fiber arrays and packaged by using metal shell. The measured on-off switch time was less than 13.4 ns and the extinction ratio was about 31.8 dB. The results showed that LNOI based electro-optic switch was an excellent candidate for developing high performance integrated optical beamforming network chips used in microwave photonic radars.