Based on the SPICE parameters model of ideal diode, a modified small-signal equivalent circuit model of terahertz planar Schottky diode was built according to the physical structure of the diode. On-wafer device-under-test (DUT) structure based on CPW de-embed method was designed according to the equivalent circuit model of the diode. The small-signal S parameters were measured in the frequency range of 0.1-50 GHz and 75-110 GHz. All the parameters of diode model such as capacitances, resistances and inductances were extracted via the test results. Comparison between DC I-V resistances, empirical formula capacitances and high frequency parameters was made. Both of the capacitances and resistances at high frequencies were different from low frequencies. The built small-signal equivalent circuit model of terahertz planar Schottky diode was validated by simulation and the results of model simulation agree well with the DUT S-parameters. Complete equivalent circuit model and the testing method can more accurately represent the working state of the device under high frequency compared with the ideal diode SPICE model and the parameters of the traditional extraction method. This robust method is suitable for Schottky diode model extraction, which is useful for further nonlinear circuit design and optimization in terahertz wave frequencies.