Abstract: In order to explore the seismic nonlinear response characteristics of seabed sites, this paper calculates the horizontal - vertical (HVSR) spectral ratio of the ground motion recorded by the S - net network station during the Mw7.4 earthquake that in Off - Fukushima on March 16, 2022. The HVSR spectral ratio results of strong earthquakes (PGA≥100cm/s²) and weak earthquakes (20 < PGA < 100cm/s²) are compared, and three parameters of quantifying site nonlinearity, namely DNL, ADNL, and PNL, are calculated to evaluate the site nonlinearity level of 148 strong motion stations. The characteristics of these three parameters are discussed, and the relationships between the nonlinear parameters of buried and unburied stations and PGA and PGV is analyzed by regression. The results show that when the horizontal PGA exceeds 100cm/s², the sites exhibit obvious site nonlinear characteristics during this earthquake, including the shift of the dominant frequency to lower frequencies and the decrease of the high - frequency spectral ratio. All three nonlinear parameters have a good correlation with PGA and PGV. The nonlinear parameter PNL is more suitable for the nonlinear identification of S-net sites, while the thresholds for DNL and ADNL to identify the nonlinear response of offshore sites need to be re-evaluated. The station layout has an important impact on the nonlinear assessment. When using the strong ground motion data recorded by buried and unburied stations to distinguish site nonlinearity, it is necessary to discuss them separately.