A high-linear bias-free polymer Y-type coupler electro-optic (EO) modulator is proposed, which consists of Y-type splitter, Y-type combiner, two-section EO region consisting of a pair of push-pull poled waveguides, and a microstrip line electrode. Device structure is proposed and its parameters are optimized. Using Fourier transformation on the applied electric modulation signal and combing the transfer function of the device, a novel formulation technique is presented to model and characterize the device′s static and dynamic performances. Formulas and expressions are derived, with respect to state function, static response, modulation response, third-order intermodulation response, etc. Calculation and analysis results indicate that, the half-wave voltage of the modulator is about 2.69 V, and the 3 dB modulation bandwidth is about 143 GHz. As the modulation coefficient is within the range of 1%~10%, the suppression degree of the fundamental signal on the third-order intermodulation distortion signals is as high as 60 dB~90 dB. The proposed theory and related formulations can also be used to the design, modeling and analysis of a similar device based on Y-type coupler structure.