Aiming at the technical problems of large volume and low bandwidth of traditional lithium niobate electro-optic modulator, a new design method of electro-optic modulator based on thin film lithium niobate modulator chip is proposed. The high-frequency transmission simulation model of thin film lithium niobate electro-optic modulator is given. The design of high frequency signal feed in thin film lithium niobate electro-optic modulator and high frequency impedance matching design of transition film substrate are introduced in detail, the key specifications of 40 GHz small size thin film lithium niobate electro-optic modulator are tested. The test results show that the insertion loss, half wave voltage, 3 dB bandwidth and product size of the designed thin film lithium niobate electro-optic modulator are 4.1 dB, 3.9 V, 40 GHz and 30 mm ×10 mm×5 mm respectively, superior to the traditional lithium niobate electro-optic modulator.