A photonic approach to generating a precisely π phase-shifted phase -coded millimeter-wave waveform with large frequency tunability based on dual-polarization modulators (PolMs) i s firstly proposed and demonstrated. A pair of complementary phase-modulated signals is generated along the x and y directions of PolM1,when a radio frequency signal is applied to the RF port of PolM1.As the modulated sign als are complementary phase-modulated ones,the phase difference between first-order sidebands along the x and y directio ns is π phase shift, obviously,the polarization of complementary phase-modulated signals is orthogo nal,and that of the optical carrier and first order sideband is also orthogonal.Then the lower side band is filtered by a bandpass filter. Thanks to the orthogonality between the optical carrier and higher first order s ideband along x and y direction s, they can be adjusted to an angle of 45° and 135° relative to the principal axi s of Pol M2with a polarization controller (PC). When a switching pulse signal is applied to the PolM2,the polarization directio ns of the two orthogonal lightwaves at the output of PolM2will exchange,and t he phase-coded millimeter-wave waveform with large frequency tunability and precise π phase shift will be realized at the output o f the polarizer.