Abstract：The research on the nonlinear dynamic process of the interaction between soft rocks and water has great values for the studying method of softening process；and it also has some practical significances for the determination of softening parameters. Based on the data analysis of the test results of white and grey colored argillaceous siltstone from the Water Supply Reconstruction Project from Dongjiang to Shenzhen in Guangdong Province of China，in different saturation times，it is found that the softening process of this kind of soft rock is provided as nonlinear characteristics. By applying the fractal analysis method of time series to the interaction system of soft rocks and water；the least needed numbers of dimensionalities (i.e.，variable numbers) for the description of the system is determined by rebuilding the phase space. The test research shows that the microstructure，the connecting strength and the roughness of the grains can be well reflecting the softening features of the saturated soft rocks，and are the key factors for softening process. So，the pore distribution fractal dimension Ds，cohesive force c and internal friction angle j are considered as the necessary variables for the description of the system. And then，the nonlinear dynamic equation of the softening process of the soft rock is established by applying the inverse theory；the undetermined parameters of the equation are obtained by approaching the time series of the three variables to the model by the method of searching for the minimal value of residual sum of squares. Thereby，the nonlinear dynamic process of the interaction between soft rocks and water can be analyzed. The researched results show that the calculated curves of pore distribution fractal dimension of micro-structure，cohesive force c and internal friction angle j of the soft rocks are very close to the test ones，which means that the soft rock-water interaction system has a distinct nonlinear dynamic feature. Meanwhile，the results also show that the model can well predict the varying regularities of microstructure parameters and mechanical properties of saturated soft rocks condition in a certain time scale.
