Abstract: In order to solve the problem that in-situ ground motion parameters are difficult to obtain, the in-situ ground motion parameters test system, which includes a mobile system, a load servo system, a dynamic response system, and a data acquisition system and has a wide range of applications, strong controllability and high reliability, was developed to carry out the test of field nonlinear shear modulus of silty clay in Nanjing and obtain the field G-logγ curve and G/G_max-logγ curve. It is found that when the γ exceeds 0.1%, the G decreases faster and faster, reflecting the nonlinearity and strain accumulation of soil under dynamic loading. In addition, the back analysis of model parameters by genetic algorithm was conducted based on the Martin-Davidenkov model to get the range of model parameters in the shallow soil layer of the test area.