Based on the records of 6 KiK-net stations in the 2021 Fukushima earthquake (MJMA 7.3),the nonlinear temporal change process of the site is studied based on the moving time window deconvolution method,the moving time window spectral ratio method and the moving time window autocorrelation function method. The threshold and degree of nonlinearity are analyzed. The results show that the stability of the three methods is different. Moving time window deconvolution method is easier to obtain a more stable nonlinear temporal change process of soil,but it reflects the change of average wave velocity from surface to underground,result of low degree of nonlinearity. For the soil with strong wave impedance ratio in the shallow layer,the moving time window spectral ratio method and moving time window autocorrelation function method can obtain a stronger degree of nonlinearity. However,for the soil without strong wave impedance ratio in the shallow layer,it is not easy to identify the stable nonlinear temporal change process by these two methods. The strongest nonlinear moments identified by the three methods are basically the same, which are located near the peak acceleration of the whole recording. The nonlinear threshold values of the six stations identified by the three methods are about 40~140gal,and the shear wave velocity drops between 16~53%,indicating strong site nonlinearity.