Abstract: After Guizhou red clay is reacted with chlohydric acid,the unconfined compression strength and goethitecontent of the clay decrease. And there is a function relationship between them. In this paper,the functional rela-tion between goethite content and time is derived from determining chemical kinetics parameters of reaction ofgoethite and chlorhydric acid The forecast for the change of mechanical properties with time has been given accord-ing to the chemical kinetics. The principle and method in this paper can also be used to prediction of mechanicalproperties of soil and rock affected by similar environment factors.