Abstract: The importance of the vertical nonlinearity of site on the earthquake response analysis has been addressed by many researchers. However, most of the related studies have focused on the horizontal ground motion, while the knowledge of the vertical nonlinearity is limited. To investigate the vertical nonlinearity of site, including the modulus degradation curve, nonlinear threshold and recovery process, the constrained modulus and compression wave velocity were estimated based on the KiK-net (Kiban-Kyoshin strong motion observation network) seismic data in this study. First, dynamic parameters of sites were extracted by seismic interferometry and soil dynamic parameter technique. Then in-situ constrained modulus degradation curves were estimated based on a currently available model. Furthermore, time-frequency analysis was used to obtain more strong motion records, hence the constrained modulus degradation model can be further verified. Main results are as follows: 1) On the whole, the nonlinear threshold of vertical ground motion is larger and has a wider range (10^-6-10^-4) compared with that of horizontal ground motion. 2) The recovery process of the vertical behavior of site is basically consistent with that of the horizontal behavior and the influence of strong earthquake on the compression wave velocity is obviously less than that on the shear wave velocity, e.g., 10%-15% difference for Tohoku-Oki earthquake. 3) The lower limit of the in-situ constrained modulus degradation curve is negative correlated with groundwater table and confine pressure. In-situ vertical nonlinearity is evaluated based on seismic data in this study and the results can be regarded as the verification of the inhibiting effect of the groundwater on the vertical nonlinearity of site.