Abstract: The rheological characteristics of incompetent rock are analysed by using U-t curves for the whole process of rheological deformation. The relationship between the rheological deformation rate p and stress a is given. The characteristic stress point of their rheological process is found on -d chart. The author proposes an approach to determine rheological stress of the crtical uniform rate rheololgical stress with ji-ij the inflection point of a curve and the principle of determining rheological strength is decribed.